Okay, let's see . . . you'd almost certainly want fighter 2, paladin 2, and sorcerer 3. Bump it up to 5 in paladin for the second attack and fill in the rest with sorcerer for spell slots and sorcery points. That makes you a 15th level spellcaster. Half-orc, 20 Str, great-weapon master and polearm master feats, quickened spell and empowered spell metamagics, great-weapon fighting style, polearm.
First round, enemy moves into reach - attack of opportunity, 1d10 +5 Str mod +10 GWM slashing or piercing, smite for 5d8 radiant damage.
PC's turn, quickened hold person. Enemy is paralyzed, all attacks within five feet automatically crit.
Attack #1, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
Attack #2, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
Second round, enemy is unable to move or act.
PC's turn
Attack #1, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
Attack #2, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
Action surge
Attack #1, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
Attack #2, 3d10 +5 Str mod +10 GWM slashing or piercing, 10d8 radiant
quickened scorching ray, 8th level
nine rays, each dealing 4d6 fire damage
Totaling
19d10, coming to 119.7 after fighting style on average +105 from Str mod and GWM, piercing or slashing damage
65d8, coming to 292.5 on average, radiant damage
36d6, coming to 153 after empowered spell on average, fire damage
670.2 average total damage (1031 maximum), potentially +5 from lame dragon sorcerer bonus or +? from two wild-magic surges. That's the best I can come up with.
That's going absolutely supernova, and it would use all your spell slots above third level, as well as 13 sorcery points and action surge. Needless to say, your damage per round would drop off pretty quickly thereafter. Now, if you could rely on an ally giving you haste and another paralyzing the enemy, you could go sorcerer 17/warlock 1/fighter 2 and get pretty insane with guaranteed-crit scorching ray and hex. I'm not going to bother to do that math right now, but suffice it to say, I think you got 'im.
famousringo, sad to say, your approach doesn't work. If you quicken a spell, you cannot cast another spell except a cantrip on the same turn, even if the quickened spell is a cantrip. Or at least that's how I read the rule. You also, by errata, are allowed only one surge per turn.