>>45383327In-game? Espeon and Umbreon are your strongest options for different reasons.
Umbreon is your "let's toxic-confuse ray-protect stall anything up to 20 levels higher that doesn't have Fighting STAB" bro. If you want to have an easy time with a legendary, a strong trainer, a champion or some of the Elite 4, Umbreon will cheese straight through. Anticlimatic, though.
Cheesed the big scary Giratina with a level 26 Umbreon in first ever playthrough of Platinum. Wasn't enjoyable after how much she was foreshadowed as a threat, especially after a viciously hard and agressive battle right before encountering her.
Espeon, meanwhile, is just a good old strong-and-fast sweeper - nothing special in competitive play, but in-game, it can be all you need in 99% of battles. The great big honking secref of most mainline games is that you can just level up a Kadabra/Espeon/Infernape/Starmie/Mismagius/nearly any legendary/Jolteon/Galvantula/heck, Dedenne, click STAB/coverage and sweep through 99% of stuff (doesn't work as well with similar physical threats, because there are far more stupid useless physical walls than special ones; though a Crobat or Aerodactyl can wreck a lot of shit for being so accessible, too). Espeon is worse than Alakazam but easier to access; better than Kadabra; and arguably better than everything else on the list save Infernape. So it's a "fair" easy modo that lets you breeze the game on sheer power but doesn't overpower level+10 opposition or competitively optimised teams (still not useless against those, but needs a HA, is prone to dying a lot and being compared to worse Alakazam/Tapu Lele etc).