Water/Ice no contest... water types only suffer from special related attacks, which amnesia can fix up. Many water types also have the ability to learn many other elemented attacks, ranging from psychic, electric and ice to even grass and fire. With such a wide range of attacks learn by water types, water type is one of the strongest in the game.
Ice type itself is very powerful except for some of the cripple-stated ice types out there like Sneasil... It's almost gurantee that there is an ice attack out there somewhere as ice attacks has such a big advantage over many other types. Freeze is THE worst status you can ever get as a frozen Pokemon is as good as dead (you have to either switch and use heal bell/aromatherapy or attack with the new Pokemon but this will result in the opponents getting a free turn, or your oppoennt can sit there to buff up against your non-moving frozen Pokemon.)
Water/Ice = the best imho
(plus they got some of the cutest Pokemon out there, like Articuno, Quagsire and so on ^^)