For players looking to mix things up, multiclassing provides some excellent possibilities to explore unique combinations of abilities.
Summary
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Dungeons & Dragons Fifth Edition has come out with many subclass options over the last several years. Within that framework, nearly every class has a multitude of options to customize their character to their desired play style. In particular, the wizard's versatile spellcasting makes them one of the game's most versatile classes.
However, this isn't enough for some players. To create something wholly unique, some D&D 5e wizard players choose to multiclass. Not every class works well for the wizard, which can easily go to twentieth level with just its own abilities and dominate a game. Nonetheless, plenty of classes can make for a powerful wizard multiclass in D&D 5e.
Updated August 16th by Isaac Williams: Wizards are more than powerful enough in D&D 5e, but multiclassing can take them even higher. This list has been updated in-line with CBR's current formatting standards and to shine even more light on the best wizard multiclass builds in D&D 5e.
12 Barbarian
Few D&D 5e classes are more different than the barbarian and wizard. The wizard's power lies in their academic study of magic and the strength of their mind. A barbarian's comes from violent Rage and immense physical prowess. This thematic clash also gives way to a mechanical one.
The D&D wizard has no synergies of any kind with the barbarian. This is because Rage, the barbarian's central feature, prevents a character from casting or Concentrating on a spell. This renders almost all of a wizard's features worthless. As a result, the barbarian makes for a very poor wizard multiclass in D&D 5e.
11 Monk
Monks are one of D&D 5e's most mystical martial classes. However, their use of magical abilities to fight in combat doesn't make them well-suited to a wizard multiclass. A monk's features rely on moving in and out of combat, making as many attacks as possible. Most D&D wizards want to stay far on the sidelines casting spells.
These twin roles don't work well. Wizards lack the hit points to be in combat, and monk features don't synergize with spellcasting. The only real benefit is some escape options for a wizard, such as Step of the Wind. However, they won't have the Ki points use these abilities often. As a result, it's an underwhelming D&D 5e wizard multiclass
10 Ranger
Wizards need a specific reason to take levels in a martial class. Their hit die and abilities will never lend themselves well to melee combat. As a result, martial and wizard multiciasses give specific abilities that enhance a D&D wizard's spellcasting in some way. The ranger doesn't have many of these.
A D&D ranger uses their magic to traverse the wild or enhance their weapon attacks. Neither of these synergizes well with a wizard, even more martial subclasses like the Bladesinger. The build doesn't do any of a D&D wizard's party roles better than a single-classed wizard would be able to.
9 Bard
Multiclassing spellcasters is difficult in D&D 5e. Even though a build with two full spellcasting classes like the wizard and bard retains full spell slots, they fall behind in other areas. They can still only learn spells up to their highest level in a class. Spellcasting classes get most of their power from high-level spells. As a result, the trade-off has to be worth delaying this.
The D&D bard doesn't offer enough to make this wizard multiclass worth it in D&D 5e. Its spells use a different ability score, meaning they will never be as powerful as the wizard's own magic. In addition, most of a bard's best supportive features only become invaluable past fifth level. This is far too late to delay spellcasting in exchange for slightly more support ability.
8 Druid
The druid and wizard are two of D&D 5e's best spellcasting classes. However, this doesn't give them much synergy with one another. Both classes rely on their spells for their power. Their spell lists lack much overlap in role and use different spellcasting ability scores. Although most wizards have decent Wisdom, it's not enough to make druid spells effective.
The druid doesn't offer a D&D wizard multiclass enough to make up for delaying high-level spells and features. A handful of levels in druid don't make a wizard more effective either in or outside combat. Wild Shape can provide some modest utility, but not enough to make it mechanically worthwhile.
7 Paladin
The paladin and wizard are very at odds in D&D. The paladin is one of the game's most martial classes, with access to all weapons and heavy armor. The wizard is the classic arcane caster who eschews weapons and armor. Most combinations of the two don't work. However, certain combinations can prove unexpectedly powerful.
Starting off with levels in paladin gives heavy armor, which is a significant Armor Class bonus for the armorless wizard. The wizard's high number of spell slots can fuel many more Divine Smites than a regular paladin gets access to. This build is somewhat impractical for many characters, but can provide a unique playstyle for a D&D 5e wizard multiclass.
6 Sorcerer
The sorcerer and wizard have been very similar throughout D&D history, to the point of sharing a spell list in some editions. However, this doesn't make them well-suited to multiclassing. The sorcerer runs into the same problems as many other classes. Levels in it are levels the wizard isn't progressing in their own spellcasting. It doesn't offer benefits to make up for this.
The primary benefit from D&D sorcerer levels would be Metamagic. This could lead to an unparalleled level of flexibility with a wizard's versatile spellcasting. However, the multiclass wouldn't have enough Sorcery Points to be worth it. Modifying one or two spells a day is a weak reason to sacrifice several levels in a wizard multiclass.
5 Rogue
Rogues and wizards are such an iconic D&D combination that they have a subclass dedicated to it. The D&D 5e Arcane Trickster rogue subclass gives the rogue limited wizard spellcasting. However, the idea can work in reverse. Wizards can benefit from a rogue's out-of-combat abilities.
D&D wizards are unlikely to use Sneak Attack. It requires weapon attacks, something that most wizard subclasses struggle with. However, Expertise increases how much a wizard can do outside combat without using spell slots. In particular, it makes them almost unparalleled with skills like History or Investigation. However, like many D&D 5e wizard multiclasses, it comes at the cost of some spellcasting.
4 Warlock
Warlocks and wizards have a lot of differences, both thematically and mechanically. D&D wizards gain their magic through arcane study and academic focus, while warlocks make bargains with eldritch beings. However, warlock levels can make for a surprisingly versatile wizard multiclass in D&D 5e.
The most obvious is Pact Magic. A warlock's spell slots refresh on a short rest rather than a long rest. This stacks with the wizard's Arcane Recovery to give them a lot of additional spell slots throughout the day. In addition, a warlock's Patron abilities and Eldritch Invocations add significant flexibility to the wizard. However, its Charisma spellcasting means many of its spells aren't suited to a wizard.
3 Artificer
Artificers and D&D wizards have one natural advantage while multiclassing. They both use Intelligence for their spellcasting and other features. In addition, the artificer has a unique rule when multiclassing. They round their level up for determining spell slots, making them less costly to take levels in than other half-caster classes.
The main thing an artificer can offer a wizard is versatility. They have several valuable utility features, such as the ability to create magic items. In addition, subclasses like the Artillerist can buff the wizard's damage or durability. The downside is that the multiclass trades power for versatility. The artificer adds variety but doesn't improve anything the wizard can do. It still delays powerful late-game spells.
2 Fighter
The fighter and wizard multiclass combination is one of D&D's oldest. In general, a wizard taking fighter levels won't become a particularly adept combatant. However, the D&D 5e fighter has one incredibly useful ability to make this one of D&D 5e's best wizard multiclass builds. Action Surge gives a character a second action a turn.
D&D 5e only has rules restricting bonus action spellcasting. A character with two actions can cast two high-leveled spells in a single turn. This is enough to enable many powerful combinations and leave many enemies reeling. The combination works better if the character starts as a fighter, due to heavy armor proficiency and a Constitution saving throw proficiency. However, it can work the other way.
1 Cleric
D&D clerics and wizards have to be very careful with their multiclassing. They both rely on their spellcasting for most of the power. However, they use different spell lists and spellcasting ability scores. A character who tries to take even levels in both will have a lot of low-level spells they can't use very well.
However, one or two cleric levels are invaluable for a wizard. Certain Domains, such as War or Tempest, give a character heavy armor proficiency. This is a significant boon for a wizard who ordinarily gets no armor at all. Other D&D 5e cleric subclasses such as Arcana Domain have invaluable low-level features. This comes with no spell slot cost, delays high-level spells by one or two levels, and gives access to magic like Healing Word.