Water type Pokemon are my favorite. They have a very good defense, and the only big threat to them is electric type Pokemon. Grass does pretty good damage to them, but Water type Pokemon can learn some Ice moves, and that can devastate Grass Pokemon. Ice is also really power against Dragon and Fire too. They also use Surf, which is perhaps my most favorite move in the game, and the most useful since you actually need it to progress. That being said, Water Pokemon are generally very useful since they're tanky and tend to be effective against the majority of the types.
I also love the Fire type Pokemon. They deal massive amounts of damage, but only downside is they're usually really weak defensively, and can go down rather quickly if you don't dish out a lot of damage before your opponent does. Especially since they're weak against water, ice and rock, and rock is very common in the Pokemon games, so I tend to stay away from Fire Pokemon.
Psychic Pokemon are also pretty fun to use since they can deal out massive amounts of damage, and can confuse their opponents for a really long time. But again, they have pretty weak defense too.