It's so hard to limit this to just 10, but here is my list I have played myself as far as fundamentals/skill wise goes:
Sonicfox (best batgirl, so many different vortexs and setups you're scared to let him get near you, and maximizes damage as much as possible)
Honeybee (best flash, has a ton of different knockdown setups and ground pound crossovers and trait shenanigans, different cool resets and tech too)
REO (all around great player fundamentally, can zone really well or rush really well, plays excellent footsies with good reactions and conversions)
Woundcowboy (best sinestro, has really good spacing, he always knows exactly where to place himself where his character can play footsies with you and deal with most of your best options)
Theo (best Aquaman, has one of the best defeneses can make himself like a literal fortress, when you finally get in trait, extremely patient)
16bit (best Catwoman, understands the game's footsies and meta very well, understands baiting wakeups and/or stuffing wakeups very well, can take your life bar with a single knockdown)
Jupiter (best martian, one of the best Zoners in the community and has very good defense at the same time. Has really awesome conversions, can combo off of any red orb or jump attack)
A foxy grampa (best wonder woman, plays footsies very well and has one of the best rushdown games, constantly in your face doing 50/50s and getting 40% off each correct guess, never drops combos)
Noobe (best doomsday, extremely good pressure, plays very fast paced hard to keep up and slow him down sometimes, very overwhelming rush down with great reactions, techs most throws and air grabs all jumps)
Gross (best green lanturn, very scary up close, has over 100 different setups and mixups and resets to cross you over or not cross you over with j3, very great footsies and corner mindgames, maximizes all damage gets 50% often)
