My top three are moon druids, necromancers, and bardlocks.
Druids: tanking (elementals), utility scouting, amazing summons (Conjure Animal: Cobras/Giant Poisonous Snake is fantastic especially if someone casts Fog Cloud/Darkness for you), Pass Without Trace, Spike Growth for terrain control, all spells known automatically (unlike a wizard), fantastic Onion Druid capstone, still has great high-level spells like Planar Binding, Animals Shapes (turn everything into elephants) and Foresight.
Necromancer: Minion-mancer like the druid with a better concentration economy, more information-gathering and miscellaneous spells.
Bardlock: Eventually even better than the Abjuror at Counterspelling due to Jack of All Trades + Peerless Skill. Incredibly sneaky with Stealth expertise, can steal a few choice spells from other classes (Conjure Animals/Aura of Vitality/Spike Growth/Death Ward/Counterspell/Bless/Circle of Power/Wish are my picks, although Circle of Power is the weakest pick there/least likely to get used), skillmonkey, good damage, good healer. At level 19 learn True Polymorph and create Nycaloths/Glabrezus who can e.g. cast Darkness on themselves to leverage truesight before wading into melee, or can ready Dispel Magic to act as a poor man's Counterspell with better range.
I could play any of the above three as a solo character and (usually) win encounters that are designed for a full party. And the ones I didn't win, I could run away from successfully.
P.S. I also love pally-sorcs. They have a fantastic action economy and I also really like the Paladin of Devotion's fluff. (Don't care much for the other paladins even though they are arguably better mechanically.) Strong melee attacks with improved smite and Mounted Combatant/GWM/Sacred Weapon, plus bonus action Mirror Image/Shield of Faith/Polymorph, plus Counterspell access, plus Aura of Vitality healing? Yes, please. There's no level after level 6 that isn't fun for this build.
I also love Shadow Monks, but I wouldn't call them the "most powerful," just fun. They do combo well with all of the above though, e.g. the Shadow Monk's concentration can go towards leveraging the bardlock's cobra's blindsight via Darkness, and the Shadow Monk's PWT-powered scouting makes it less likely that the necromancer's skeletons (etc.) will run into something unexpected that makes them have to run away and reconfigure.