I love the Eeveelutions Espeon and Glaceon the most. I would expect Umbreon to be a favorite for me but surprisingly it's not.
I also have a soft spot for Jolteon.
Electric Pokemon have interesting move sound effects.
Some of my favorites (not in order):
1. Whimsicott (dunno why)
2. Froslass
3. Dog-like Pokemon such as Houndoom, Electrike, Mightyena, Zoroark
4. Chandelure
5. Aggron
6. Magnezone
7. Lumineon
8. Togekiss
9. Metagross
10. Honchkrow
I would have an entire team of favorite Pokemon just for their awesome cries, too. This could be weird but Pokemon with intimidating cries like Metagross, Aggron, and Hydreigon sound amazing when you send them into a single or double battle. ^^;