All the bosses can be hard if they are used right. However some have more potential than others. Here's what I think.

Robot: The easiest, no question. Especially if you shoot it with ice. Otherwise spaz can make quick work of it with his kick. The only way to make this boss interesting is to limit the player somehow. Like by using a frog morph, or by filling the room with respawning enemies. But even then the boss dies faster than you can lose 5 hearts.

Turtle boss: By himself, probably the second second easiest to defeat. His attack can get you if you're not paying attention, but it's easy to dodge, though it can't be stomp shielded (use the first few frames of the stomp animation which make you invincible) very easily. The thing that makes this boss really easy is the fact that spaz can just kick him to death at point blank range. This boss is fun to use as a "support" for some other boss though. Stick him in some unreachable place and let him harass the player by throwing things.

Crab boss: Has no real weaknesses, but no real attacks either after the first phase, which ends quickly. The only way this boss can hurt you is by flying into you, and it's easy to dodge if there's room. That's really the key. To make this boss difficult, the arena has to be difficult. Fill it with spikes, make it full of respawning badguys, make the player fight on a tiny platform with death pits to the sides. Or do something with water, since it fits the theme. Overall it has decent potential, but the real challenge to the player will have to come from other things.

Bubba: His fireball is easy to avoid, but the tornado is quick and sometimes hard to avoid. It also has a weird knockback effect. Again pretty easy by itself, but with other hazards it might pose a challenge, though spaz can kill it somewhat quickly. Stick bilsy on a platform below the room for extra fun. He will help out with his fireballs. Bubba can also be used as a "helper" for some other boss, like in cooba's example, but personally I don't like that example level very much. Maybe it's just because he hid them with lighting effects though.

Bilsy: Some people think this is the toughest boss. By himself he's actually very easy since his fireballs can be stomp shielded once you have the timing down. They do a lot of damage but it's not a big deal if you know how to block them. Harder is probably the fact that he teleports around, making spaz's death kick useless. Personally I think bilsy is much more useful as a helper, using his fireballs to make another boss harder. Same as the turtle boss.

Bolly: This is a fun one. The missles are easy to dodge but can't be blocked by stomping. The chain is easy to destroy but still easy to get hit by too. And this boss tends to die fairly slowly. It's really not THAT hard, but it's a good boss to use in a room full of other hazards. Better than the crab boss or bubba in my opinion. The only thing is it's easy to do a lot of damage by stomping on it over and over, so I recommend using celing spikes. Creative people can also use this boss as a sort of missle turret by trapping it with wind events (or is it belts?) and destroying the chain with exploding TNT.

Queen: Easy in the single player, but can actually be very hard. And can take a very long time to kill depending on level design. Like most bosses this one is all about augmenting the difficulty with other hazards. Even the official level this boss is in does it. Also you can have a lot of fun with this boss by making it move left on belt events. It will cause autoscrolling, although a really weird sort that has little gameplay use.

Devan: Hard because he has lots of health. But easy to avoid. Like so many others, you need to make the room itself hazardous. You can also climb the level by stomping on his second form, so it might be necessary to prevent that through spikes, or maybe lighting.

Rocket Turtle: My favourite. And the hardest, no question. It's all about the level design with this one. If he makes it to the end of the level you lose, and if you kill him before makes it to the end, you win. And along the way a bunch of stuff should ideally make it as hard as possible. This boss also flies at a different speed depending on difficulty level. Hard mode usually ends up being REALLY hard.