Currently I'm using Entei in my team and it's a disaster; it won't learn Flamethrower until level 51 and until them I'm stuck with pissy Ember.
Not that it's going to matter in any post gen 3 games. Enter is a physical sweeper.
Although I love fire types, Enter certainly is not my favourite. Here's the stats of the beasts:
Raikou (user of mainly special attacks with Electric attacks)
HP - 90
Attack - 85
Defense - 75
Special Attack - 115
Special Defense - 100
Speed - 115
Best speed of the three.
-Weak to common attacks.
Entei (user of mainly physical attacks with Fire and mix attacks)
HP - 115
Attack - 115
Defense - 85
Special Attack - 90
Special Defense - 75
Speed - 100
Very versatile.
- Lacks a great physical STAB move with Fire Fang being its best physical fire-type move, and flare blitz entei is kinda rare.
Suicune (user of defensive moves with Water and/or Ice moves)
HP - 100
Attack - 75
Defense - 115
Special Attack - 90
Special Defense - 115
Speed - 85
Great defenses, especially when using Calm Mind in synergy.
-Very slow to ultilize its strength.
Personally, I think Suicune's the best because of its defenses while at the same time using Calm Mind to get stronger, becoming an effective wall but it's up to you. If you want a defensive pokemon, gowith Suicune, or if you want a special sweeper pokemon, go with Raikou, or if you want a physical sweeper, go with Entei. Yeah, I should have answered this keeping gen 2 in mind, but who cares anyway..