Huh, well, I don't have a solid top 5, but I have a few strong ones I like:
- Glaceon: This Pokemon has some pretty neat combos. The abilities work great with Hail, Blizzard is unstoppable with 100% accuracy and STAB, and Galceon's EV trained Sp. Attack is insane. 'Nuff said.
- Gengar: A notoriously famous Pokemon used in competitive battles, the options are expanded even further in Gen V. Step 1: EV train Sp. Attack. Step 2: Teach it Hypnosis, Dream Eater, Hex, and Nightmare. Step 3: Let the destruction begin.
- Starmie: Another old school Pokemon that is great for the Battle Tower or whatnot. With an EV trained Sp. Attack, Surf (which also gets STAB), Ice Beam, and Thunderbolt will cover a lot of types for you. And of course Recover won't hurt in tight situations.
- Machamp: Finally, a physical sweeper. This guy has equally insane Attack as Glaceon and Gengar have Sp. Attack. Make sure it has No Guard as the ability, and EV train accordingly. Teach it Dynamicpunch and Cross Chop (for both 100% accuracy, thanks to No Guard, and STAB). Have Earthquake and Stone Edge as backup. You now have a killing machine.
- Pseudo-Legendaries: If trained right, these things can top most other Pokemon. There's too many to go through one by one, but if you know about Pokemon enough then you won't need a full explanation anyway.