I think it's the Twins, because the others are pushovers.
The Destroyer is extraordinarily simple. You can either just fly back and forth, spamming a weapon that creates lots of projectiles such the Daedalus Stormbow or the Meteor Staff, or you get a piercing weapon and shoot down his body, destroying him in seconds. The laser spam may seem dangerous, but can be evaded easily by moving horizontally. The probes may be a problem if you let them become a big swarm, but they usually end up just being extra hearts.
Skeletron Prime is at least a bit more complicated to dodge than the Destroyer, but is still very easy. The saw is extremely easy to dodge, you can just ignore it. The vice is a bit more dangerous, but it still barely poses a threat. The cannon is deadly if you actually run into the explosions, but if you build the right arena and have the right mobility, the bombs won't be able to get near you. The laser is the only arm that actually poses a threat, but can be destroyed quickly. The head is a joke, because it doesn't even attack you part of the time, and the rest of the time it charges at you, which can be dodged by simply moving backward.
The Twins, in my experience, are trickier than the other mech bosses, but are still somewhat easy. Spazmatism's flame balls can be dodged by just moving up and down, but can still be run into accidentally because of their slow speed. In his second phase, he moves extremely slowly with the flamethrower active, so you can just back up slowly, and his charges can be avoided by flying up and down. Retinazer's first phase is pathetic, because he infrequently shoots lasers, but his second phase can be pretty difficult, because of how fast he fires his lasers.