EzekielRaiden
Follower of the Way
The problem with Sorcerer is, ironically, precisely the opposite of the problem with the Wizard, and why my vote will always be "Cleric is the simplest, but no full spellcaster is simple."Sorcerer.
Everyone who said warlock is wrong. It's a very complex class that's only simple if you know what you're doing. It's very easy to cock it up and miss taking Eldritch blast if you're new.
A lot of moving parts. Sorcerers more self contained but you would almost have to go out if your way to mess it up.
The Wizard's problem is that it has this ENORMOUS toolbox, but a crapload of those tools are just...not very good a lot of the time. So it requires extensive charop knowledge in order to reach its maximum potential. But, ironically, once you have a basic knowledge of what spells are Generally Great (and there really is a shortlist of Generally Great spells available to Wizards), the Wizard is actually a pretty blunt instrument: Cast More Spells.
The Sorcerer's problem is that it has a MICROSCOPIC toolbox, but a crapload of ways to tweak and fiddle. This also requires extensive charop knowledge, not because you need to know how to maximally leverage the vast arcane estate at your command, but because you need to guarantee that the minuscule pool of spells you can draw on will definitely be useful all day, ideally in ways that play nicely with your metamagic options. Those very metamagic options also pull the Sorcerer sharply away from simplicity for the same reason Invocations pull the Warlock away from simplicity: anything that lets you rewrite the very spells you're working with is necessarily a complexity booster, because now any given spell isn't one spell, it's two to five.
Cleric averts both ends of this. It has a moderately-sized spell list, slightly smaller than the potential list of Sorcerer, but importantly, you have access to absolutely all of it (up to your highest slot, natch) at the start of any given day. You don't have any fancy tricks you can pull with those spells, no opt-in features which rewrite them, no need to survey the field to ensure you're picking the clever (=Wizard) or guaranteed-useful (=Warlock/Sorcerer) options. And, because the divine list is IMO rather better curated than either Sorcerer or Wizard (Warlock is also better, but too complex for other reasons), it's usually not that hard to pick one good support spell per level, one good offense spell per level, and whatever else tickles one's fancy until you hit your limit.