Guess I'll get the eeveelutions out of the way. WARNING: May contain slight bias.
Jolteon
Tier: A
Availability: Late
Movepool: Meh.
Stats: Ninja
Comments: Hits hard and hits fast, and because of that you'll hardly need move coverage except for grass and ground types, in which case you can use Shadow Ball, or if you're lucky, HP Ice.
Vaporeon
Tier: A
Availability: Late
Movepool: Meh.
Stats: Tank
Comments: Decent bulk, absolutely amazing in the rain if you have hydration. Great support Pokemon with Wish etc. but is very capable offensively too with moves like Aurora Beam, Shadow Ball and Water Pulse (still waiting on the Scald TM!).
Espeon
Tier: B
Availability: Late
Movepool: Meh.
Stats: Good
Comments: Similar to Jolteon, but has less coverage since Psychic moves don't really work well together with Shadow Ball. Still a very solid option.
Leafeon
Tier: C
Availability: Late
Movepool: One to forget
Stats: Oh god why
Comments: Best stats in the wrong places, movepool seriously lacks coverage. Sure you could sleep Pokes with Grasswhistle (which misses a lot) and set up with Swords Dance and spam Leaf Blade but nobody's got time for that...
Umbreon
Tier: A
Availability: Late
Movepool: Decent
Stats: Very Good
Comments: This thing will not die unless you're trying to kill it. If you run a Curse set with Payback, you can dish out serious damage with enough setting up, or you can go for a stalling approach and have moves like Toxic, Protect and Wish. That being said, it takes patience to raise Umbreon because battling with it is never straightforward.
Flareon
Tier: C
Availability: Late
Movepool: Ehh...
Stats: Nope nope.
Comments: Similar case to Leafeon, really. Great attack stat, but moves let it down (i.e. aside from Fire Fang, no STAB).