Empoleon
Piplup > Prinplup > Empoleon
Water Steel is a fantastic defensive type with 10 resistances and 3 weaknesses. Ground is a rather common type for coverage though, so it's best to look out for it. You wouldn't really want to get hit by a random earthquake, wouldn't you? Although, it probably won't OHKO Empoleon even if it crits nowadays.
Stats:
Does anyone smell bulky water type? Well I do. Empoleon is a fantastic bulky special sweeper or simply a bulky wall. Its low speed doesn't really matter, but you can still boost it if you like with moves like Agility.
One unfortunate aspect about Empoleon is coverage where it lacks any real options. It has its STAB covered pretty well with Scald, Surf and Flash Cannon, but it doesn't have access to anything else other than Ice Beam and Grass Knot. A physical build is also possible but not very recommended as Empoleon's ATK stat is rather mediocre.
All these offensive moves might make you forget that Empoleon got some pretty decent utility options as well. Stealth Rock is probably the best move in this regard. However, Empoleon also has access to moves such as Roar and Rain Dance; I don't really recommend the latter though.
Abilities:
- Torrent: a Water-type move's power will be increased by 50% if the user has less than or equal to ⅓ of its maximum HP remaining.
- Defiant (hidden): boosts ATK by 2 stages when a stat is lowered by the opponent.
Torrent is the typical ability for every water starter, and it has never been good because it is too situational. Defiant is a decent ability, but it's completely out place in a pokemon like Empoleon that doesn't rely on physical moves. Actually, Defiant can work against you if you get confused, so in this one rare case Torrent is the better ability.
Conclusion:
I am of the opinion that all Gen IV starters are very good pokemon, and Empoleon is no exception to this. Good bulk alongside nice utility and good Sp. ATK makes Empoleon the most versatile of all 3 starters. Besides, it's a penguin, I have to give it extra points for that. :P