I hate the Cardassian Galor or Keldon, the one that spawn an army. I hate the borg insta kill in STF, because it's simply frustrating. Also I hate an hirogen ship, I think, one that use ecm jammer or something like that for 10s... I hate the Tholian, because they use a lot of CC. Their web is annoying and the random teleporting is not fun.
I like to fight Romulan, because they can be a tough opponent at lower lvl, and I love their ships. I like Klingon to, just for fun.
Ground, no opinion, I'm not a big fan of ground fight, and they all look the same to me.
In space I like the Borg, since they're the only enemy that can put up a fight without outnumbering players 20 to 1 and hate the tholians for their weapon disables.
On the ground, I like the regular enemies (the ones that shoot you with guns or hit you with swords) and hate pretty much everything else, especially the ones with magic shield-ignoring psi attacks and the ones that disable you.