my two cents
what are you gonna do with him? first liner defender of the weak persona or a "generic purpouse" good guy?
if you want to hit first line and overall stand between the squishy and the BBEGs, paladin is your best choice, but paladins have some downsides that can easily be fixed by dipping 2 levels of warlock (in my meta, warlock 2/ anything else is my go to dip for anything really).
Standing as a second liner, your best bet is a blaster (sorcerer comes to mind) but if you go for a pure caster class, dipping isn't adviceable (especially sorcerers) due slot progression halting mostly (which might be slightly endured better if dipping at least warlock 2/3 but only if you need a plethora of low level slots)
Steady damage dealers are better than nova ones, in my books, so if you have UA material at hand, please consider the ultra cheesy meta: hexblade warlock and hand crossbows + sharpshooter and xbow expert. If you can reliably take advantage to offset the -5 atk from ss (battlemaster's trip, for instance or a team mate that reliably shoves prone your targets), those are 30 damage per round applied straight from a feat, for an average of 57 damage per round (before adding hex/hunter's mark).
how many levels you think you will gain? are you starting from a certain point or is this some long therm agenda?
if you are going for a level 1 to level infinity type of game, if you plan to MC be advised that your character will be subpar until it hits certain breakpoints (explained below), if you start from a fixed level there is some sweet wiggle room to fix your game before even starting.
With booming blade/green flame blade, rushing "extra attack" is no longer a priority (one longsword swing is 1d8+stat, level 5 BB/GFB is +1d8 elemental damage + extra - you get some you loose some) but since you will get that eventually, you should remember that cantrips are not replaceable but by the time you start hitting twice per attack action, they won't matter that much anymore.
Best breakpoints are:
warlock 2 (hit almost immediately because of sick versatility), which provides:
eldritch blast (best blast in game) and booming blade (i usually avoid GFB because 70% of monsters resist fire);
2 1st level slots per rest (usually 1 long and 2 short per day mean 6 1st level slots, 6 hexes, 6 2d8 smites, 6 healing elixirs, etc)
2 eldritch invocations: best in slots are
agonizing blast if you specialize in blasts,
fiendish vigor for 8 temp hp per combat and
Devil's Sight aka 120ft magic darkvision, which you will miss if you go with human variant (plain and simple the best class to minmax since it provides you a feat and two +1 of your choice, which becomes three +1 if you go for specialist feats such as resilient or heavy armor master).
Paladin 6 (any spec) (Oath of Vengeance "tier 2 end of the build" breakpoint) for the +ST aura, since you will probably suck hard on dex and wis (especially if you start your array with 15 str 15 con and 15 cha)
Should be rushed if your DM loves his casters, not a priority if your DM runs Adventurers League legit modules/hardcover.
Oath of Vengeance "tier 2 end of the build" breakpoint because going further down the Paladin line has little to provide. You could move half your movement when you proc an attack of opportunity, yes, but why bother? Yes you could reach Paladin 8 and get the second ASI but is it really that important? Most of the builds won't need a 3rd feat (i'm implying that you went human variant, of course) and while I will totally agree that hitting 20 in a stat is a priority, I'm more of a "survival of the fittest" type of gamer and will take Invocations over +2 in a stat any time of the day.
Paladin 7 (Oath of Ancients) for the "resistance to any spell damage" which means that if the source is a spell (not just a slotted spell, but any "x/day" ones or magic items uses too), you get to resist its damage, even psychic or untyped one. If the spell's source has elemental affinity (PVP comes to mind)
it won't work because yours is not an elemental resistance
Paladin 8 (The "tier 2 end of the build" break point for any non vengeance paladin). Yes you could go for paladin 9 for 3rd level smites and spells but they are non essential in tier 2 games. Yes you could rush Aura of Courage at level 10th, but why bother? frighten is not a permanent debuff and is still subjected to Will STs. Monsters' DCs usually vary between 12 and 17 and provided that you have proficiency in Wis ST and Aura of Protection, you should stand around a +5 will ST (wis -1, prof +3 cha +3). If I play Tier 2, I won't ever get over Paladin 8, It really is a subpar choice.
Sorcerer x I'd never MC a sorcerer or any other "the highest slot you can achieve, the better" type of class. Exception made for Life clerics, whose characters I usually dip in the Warlock 2 sauce due the fact that Life cleric's Disciple of Life performs way better on low level spells than high level ones (cleric 3/warlock 2 has 10 1st level spell slots, just saying).
Best starting points are:
Paladin 11 (any) for tier 3 games, Improved Divine Smite.
Paladin 9/Warlock 2 (also Paladin 8/Warlock 3 works fine) for tier 3 games, yes you are going to fill the gap to paladin 11 and it will feel as long as eternity, but if your DM loves dungeon crawling, traps and darkness, Eldritch Invocations are a must.
Paladin 3/Warlock 2 for tier 2 games, you won't need to attack twice per round since BB/GFB are in play, the more 1st level slots you have, the best survivability you boast.
Paladin 1, if you must start from level 1, best class to start with is the one that provides you highest AC, HP and overall survivability
Bonus track:
Ultra cheesy build: Paladin x, Str 13, Hexblade warlock dip, hand crossbow, sharpshooter and crossbow expert feats. You won't smite with a ranged weapon but you can employ Hexes/Hunter's Marks instead. You won't even need paladin class for this build to perform (actually Battlemaster is better but you will miss Auras, mind you).
Close Quarter Shooter build: Paladins originally can't pick anything ranged combat related, UA's CQS is available though, which means that you can pick this one as your Fighting Style and spam Eldritch Blasts in melee. War Caster feat and Sorcerer's quickened metamagic mean that you can spam several EBs per round... 1-4 to 4-16 beams consistently (for the reaction one you might need to rely on some teamwork though).
