Multiclass
There are several reasons to multiclass IMHO. The most likely ones are.
1. Power.
This one is self explanatory. You want a more powerful PC.
2. Concept. You might have a concept that is not served well by the existing classes. The arcane gish for example is not that good in PHB RAW for a variety of reasons while some sort of skirmisher/scout might be better off as a fighter/Rogure than a fighter or Rogue for example.
3. Versatility. Strength based PCs tend to suck at range due to the way the rules work about throwing weapons. Or being strength based and having to resort to a bow while Mr Dex based with sharpshooter laughs at you.
4. Roleplaying. Kind of similar to concept but you don't care to much about the mechanics you really want that Gnomish Paladin with a rapier/ rogue MC.
The Builds.
The Sorlock
This one is infamous on the forums. I'm not to sure who came up with it, I know it wasn't me but I have seen it in actual play and refined it a bit. The way it works is you start the game as a Sorcerer for the saving throws and MC into warlock at level 2 and 3 and from level 4+ you take levels in Sorcerer. This build switches on at level 3 and is the spellcaster version n of the -5/+1- feats as you are marrying eldritch blast, agonizing blast+ hex to the Sorcerers ability to quicken cantrips for cheap. By sacrificing spell slots for sorcerer points eventually being able to quicken eldritch blast more or less at will at least in any reasonable adventuring day. At low levels 1d10+3+1d6 damage is decent at higher levels that is per beam, and then you quicken the spell. Since you are basically an archer you can devote a few more of your sorcerer spells towards utility and non combat options. Also having 9 level 1 spells per long rest or so by level 3 is not that unusual. That is a lot of hexes and shields you can use. Obviously a top tier damage dealer, you are charisma based for the social stuff and in some ways you are potentially better than a single classed sorcerer at non combat stuff as in a real game you probably won't sacrifice all your spell slots for sorcery points. Tier 1 build from level 1-10 and tier 1 or 2 11+ depending on how you build it as you might take 2 more warlock levels for the ASI and some non combat utility (due to overkill).
The Skill Monkey
There are 18 skills in 5E, how would you like to be proficient in all of them? Here is how you do it. Start as a variant human. Start as a Rogue and take the skilled feat. Highest score is charisma+ 14 dex and con.
Rogue 4 skills
Back ground 2
Human 1 skill
Skill 3 skills
10 skills level 1.
Then you multiclass, what order is up to you but you want these ASAP.
1 Level of knowledge cleric (2 skills from a list of 4)+ medium armor take guidance.
2 levels of warlock (agonizing blast and Beguiling influence), add deception and persuasion skills
That is 14 skills. Take 3 levels Bard and be a lore bard. You get 3 skills for being a lore bard, + 1 for the Bard MC. You will also have expertise in 4 skills. Level 7 proficient in all spells, medium armor and you are tier 2 in damage due to using agonising blast. Being charisma based and proficient in everything you also make a decent archer. The downside is no feat until level 8 (4th bard level) and then you might take 2 more warlock levels for another feat at 10. Overall a tier 3 class early on, but you are basically good at all the skills, have a lot of spell slots to put hex in (cleric+bard_ warlock levels), and eldritch blast scales for just being a cantrip. Good at all the pillars, you're not excellent at anything however. Overall anywhere from tier 3 to 1 YMMV.
The Sorcadin
This one is very easy. Level 1-6 or 1-8 as Paladin, MC into dragon sorcerer for the rest of your career. Basically you want the aura and maybe a feat, then accelerated access to spell slots. Takes a while to get going but you are never really bad as you're a single classed Paladin. A single classed Paladin get level 5 spell slots level 17, you get them at 12 assuming you go with a 6/6 split. Sorcerer spells and cantrips give you ranged options that a Paladin usually struggles with and Charisma is a primary or secondary stat.
Hexadin
This one is similar to the Sorcadin but you are dipping 2 levels of warlock for the usual suspects (blast, hex etc) but are going hexblade. This is so you can key everything off charisma and you area lot better at ranged attacks as well. Charisma is your primary stats, 14 dex and con with the default array and tank around in medium armor if you don't want to be slowed down. You will take the hexblade levels fairly early on but if you want heavy armor Paladin levels 1st. Not to bad but might be weak level 5-7 as you really want to get your aura up. You melee, ranged and spells all key off charisma and you have the aura. Even better than the actual Paladin, it takes a few levels to switch on generally around level 8-10. If that trade off works for you you end up with a better character than the actual single classed Paladin and a lot better at range. Tier 1 or 2 once it all comes together, varies by how you build it and what oath you pick.
Bladelock.
Yet another warlock abuse. The hexblade has obsoleted this one a bit but it used to be Fighter 1, then MC into warlock (bladelock+ fiendpact). It still has some advantage of the Hexadin and single classed hexblade and that is heavy armor and the fighter is front loaded as you get better saves, heavy armor, a combat style and a minor healing ability with your 1 level dip. However this build switches on very early (level 3), and its a lot smoother with the only bump at 5 where you miss out on your 2nd attack. You end up less powerful unless you key off charisma but might have more versatility due to more levels of warlock. With armor of agathys and fiendpact you are very tanky even compared with a single classed fighter. Tier 1-2 depending on the levels and the warlock part.
Fighter 1/Rogue XYZ
Two ways to build this.The 1st is your fairly typical rogue into fighter, high dex and dual wield getting two weapon style so you get dex to damage with your off hand. The other way of doing it is going strength based, start as a fighter, MC into Rogue and use heavy armor and a shield picking up the shield master feat. You are basically a brutal Rogue from 4E, The Brutal Rogue probably wll want 5 or 6 fighter levels for multi attacks, knock em prone and sneak attack away. The 1st option can be enough to drag a Rogue up a tier early on as you deal more damage in combat so depending on how you build the social and exploration aspects of the character you're a lot better at combat.
Rogue 8/fighter (champion or battlemaster) or Ranger (Hunter) XYZ.
The Rogue is a bit underwhelming at level 9+. This fixes the damage part of the character at least as 5 levels of Rogue or Ranger offers a lot as both classes are front loaded. You can also combine it with the above idea to splash a fighter level early and take the 2nd level at 9. Dual wielding you will end up with 3 attacks a round, and with Ranger abilities perhaps 4 via hordebreaker. The fighter one is more combat focused, the Ranger is also competitive however. Either way the more attacks you make the more chance you have of sneak attacking. By the time this comes online it won't drag you up a tier but it will stop you falling to tier 4 level 11+ IMHO.
Eldritch Knight (MC) Fighter EK 6 or 8, Abjurer Wizard XYZ
You take the 6 or 8 levels of Fighter 1st so you can get the feats. Then you MC into a wizard probably the Abjurer. You''ll probably want the warcaster feat. You might take an extra 2 fighter levels perhaps for war magic as you won't be taking 11 levels of fighter in any event. This one switches on fairly early and the trade off is more damage at higher levels and less hit points. You pick up a lot more spell slots which means more versatility as you probably want to focus on utility, buffing and defensive options. Since you cant use your spell slots for smiting you probably want to up cast a lot. By level 10 for example you can have level 4 spell slots, but only level 2 spells. Haste=action surge from level 11 is also an option. This is generally a tier 2 or 3 option and at higher levels is a strong tier 2 character.
