Scarf Latios. What a great revenge killer. You can switch it in to most attacks thanks to its phenomenal typing and ability, and then wreak havoc. To name a few things it outspeeds and counters, it easily takes care of Deoxys-S, virtually all dragons at +1 speed, Shell Smash Cloyster at +2 speed, Agiligross at +2, Scrafty at +2, Volcarona after Quiver Dance, the entire Scarfed metagame, and everything slower than the stuff I just mentioned. So, basically everything but Venusaur and Rock Polish Terrakion.
I usually give it Energy Ball for the surprise factor, and because a good Grass-type-user is hard to find these days. Only Ferrothorn and Virizion are used these days, and they can't do anything without setup. Plus, it makes Latios a great counter for Rotom-Ws that think they outspeed me, but don't. I also give it the amazing power of Draco Meteor, which, with STAB, still has 105 power even after the -2, so it by no means makes Latios set-up fodder.
If you're creative, don't copy and paste this into your signature (Started by 3.14kachu).