Now that I have finally defeated all of the Mechanical Bosses, I can put in my 5 cents.
The Destroyer: Definitely the easiest boss out of the trio. The only thing you have to worry about is the drones that constantly charge you, those are a pain. With a Daedalus Stormbow, some holy arrows, Orichilcum Armor, and a Nimbus Rid, he can be taken out in about a minute. All in all, he is definitely the easiest boss as long as you have the right equipment.
The Twins: Relatively difficult, but not overly difficult. With a Hallowed repeater, Daedalus Stormbow, and a Megashark with Explosive Bullets, he can be defeated, but there is one thing that is very much needed, an arena. If you don't have an arena, the match will be far harder. Taking out one of the twins at a time is obviously the way to go, it makes the match quicker, and quite a bit easier. But again, without proper equipment, an arena, and a strategy, he could very easily become on of the hardest Mechanical Bosses to beat.
Skeletron Prime: Without a doubt the most time consuming of not only the Mechanical Bosses, but of all the bosses. Preparing yourself with limited time would probably be your utter downfall, being one of the longest bosses to defeat up to this point, time is necessity and without it, it would make this one of the most difficult bosses to defeat. The weapons of choice would definitely be the Megashark with either Explosive Bullets or Crystal Bullets, Daedalus Stormbow with either Holy Arrow or Hellfire Arrows, and a Hallowed Repeater with the same choice of arrows. Along with The Twins, an arena is a necessity, trying to fight him on a Skybridge doesn't particularly make the fight any easier, you would still probably have to use a legitimate arena in order to fight him. Personally, I didn't find him to be of much difficulty, I never got below 200 health (I had to use 2 greater healing potions and dip myself in a pool of honey every now and then), but the amount of time it took to take him out was a bit of a deal breaker for me.
So overall, who was the easiest boss, and who was the hardest? The easiest was without a doubt The Destroyer, with proper equipment he can be taken out in under a minute. However the hardest boss to defeat, is a bit tricky. I would have to go with The Twins considering the fact that they were a bit difficult to dodge, even with a fairly large arena, and the fact that once they reached the second stage the damage output increased quite a bit, especially with Spazmatism's spin off of the flamethrower they became really difficult to dodge, and still be able to knock them out. Skeletron Prime wasn't overly difficult, it just took half a century for me to take him out. I can't say exactly how long it took me to take him out, but I would have to put my money on around 5 minutes. His attacks were fairly weak, plus his cannons grenades were almost completely negated if you were standing on wooden planks, so that wasn't much of a challenge, plus you can almost entirely render him useless if you work on taking out his weapons first, which after about 4 minutes in I did, which made taking the rest of him out a walk in the park, just stay a few yards back and take the Light Disks to him. My final verdict on each of the bosses is, The Destroyer, proper equipment, The Twins, Running and dodging plus assigned attacks, and Skeletron Prime, take him out as quickly as possible.
I am also going to add a Poll for the most difficult of the Mechanical Bosses to defeat, so please check that out and participate if you would, much thanks!