The fully evolved starters eligible for Gen 8 OU are Rillaboom, Cinderace, Inteleon, Venusaur, Charizard, Blastoise, Sceptile, Blaziken, Swampert, Decidueye, Incineroar, and Primarina. Of these, Smogon has sample sets for Gen 8 OU for Rillaboom, Cinderace, Venusaur, Charizard, Blaziken, Swampert, Incineroar, and Primarina. Let's compare them.
Venusaur can be a potent Pokemon in sun with Chlorophyll. However, it has numerous weaknesses. It's in UUBL, meaning it's not quite OU-level.
Charizard can perform well under sun as well. It's a powerful Pokemon, but if it doesn't have sun to help it, it's just about crippled. Its Rock weakness doesn't help matters. Its usual tier is RU, a far cry from OU.
Rillaboom is one of the few starters that's actually in OU. It has a great Ability that complements Grassy Glide well, giving it priority. It functions as a nice wallbreaker with access to Swords Dance, Knock Off, and other solid moves.
Cinderace is very versatile. It can perform an offensive role, a setup sweeper, or a pivoter. This versatility makes it an excellent Pokemon to use, as you can do a lot with it. It is fairly frail, however, and is weak to Stealth Rock, so Heavy-Duty Boots are just about required if you're using it as a pivot.
Blaziken has always been good, and it probably always will be. Swords Dance and Speed Boost make for a very potent sweeper. It's able to use excellent moves like Blaze Kick and Close Combat, making it a very potent attacker. Its relative frailty, plus the fact that it's in UU, means Blaziken isn't the best up in OU.
Swampert is a very good defensive Pokemon. 100/90/90 defenses give it impressive bulk, plus it only has one weakness. It can set up with Stealth Rock, block Explosions with Damp, pivot with Flip Turn, Toxic stall an opponent, and potentially cripple a sweeper with Roar -- all in the same set.
Incineroar can do well as a crippler. It gets access to Intimidate, Parting Shot, Knock Off, and Fake Out, plus moves like Flare Blitz. It functions pretty well in OU.
Primarina is a decently bulky special attacker. It's able to fire off attacks like Hydro Pump and Moonblast, which are boosted even more by Calm Mind. Its Defense is not the greatest, however.
I believe it's safe to say Rillaboom is the best starter for Gen 8 OU. Cinderace and Swampert also perform well in the tier, however, and some of the other starters have niches. Hope I helped!