Guardians :
Most difficult :
EMP is nasty if it catches you off guard, otherwise starship disassembler and Artillery because they hurt my high-value starships.
Least difficult :
Zombie, Vampire, Raider
Bonus ship:
Most difficult:
Zenith Bombards because they're just so good at sniping my golems and such. Electric Bombers are nasty in high numbers as well.
Least difficult:
Things like Microfighters or Shredders that get their strength in numbers. The AI doesn't correctly take advantage of their swarming capabilities.
AI type:
Most difficult:
Turtle, despite being an "easy" AI Type, because of SO MANY EYES. It gets rather tedious after a while. They're everywhere. Raid Engine is a toughie too for obvious reasons.
Least difficult:
Mine Enthusiast, because with a little caution you won't take too much damage from mines. The only real problem is the EMP mines, which can be unfortunate to encounter at a bad time. I haven't played against a lot of other AI types, but it seems like