Regice was my least favourite Regi for a long time, but then I realised that, with Rock Polish, it could achieve a decent Speed and sweep offensive teams or simply demolish defensive teams with its great coverage.
Another of these legendary Pokémon I did not like at first was Cresselia. Yes, pretty big defence, but, apparently, nothing else. But, in singles, if given HP Fire it can actually be a nasty surprise and still be a good wall.
And I grew attached to Raticate due to Joey at first, its ridiculosuly fun sweeps on XY on-line later.