It would be nice if we stated reasons along with it so here I go, from worst to best.
Devil May Cry 2: As it's the worst game in the series(although it IS good, just not good enough to have "Devil May Cry" as its name) it is the most difficult to decide a boss from. The most difficult would easily be Trismagia, but I can't say it's my favourite. The boss with the best story behind him would be Bolverk but then again, he's somehow annoying. So for me, my choice is the Despair Embodied. It's one of those battles where you fight someone who has almost the same size of you, which I greatly enjoy. Second, he uses fire, one of my favourite elements for a bad guy. He flies, changes gene(well THAT's weird
) transforms his body to whatever lethal weapons you can think AND from what I've heard can be encountered in a Secret Mission, in which you fight 3 D.E. at the same time. That would be awesome if you're a good DMC2 player
Devil May Cry 3: Easily Vergil. Swift, dark, sword user. And a Darkslayer. Simply the opening gave me great relief as we saw that magnificent battle between the two, just because we would be able to surely play that battle. The second battle was great as well. Vergil and Dante can use different Devil Arms now and transform into their inner demons. (Although the demon forms were purely made in my opinion). And the 3rd battle is epic. We have one mission only to fight Vergil, which is awesome. In there he uses some pretty neat skills, that make him look wicked and cool^_^ Also he is the ultimate rival and alter image of Dante, which gives him even more coolness, so as I already said, it's an easy choice!
Devil May Cry: Well, for the other two games, I basically chose the last and strongest bosses(although strongest is a characteristic that varies with the player, since I said that Trismagia is the most difficult for me). But now I won't say Mundus, since it was a disappointment for me. Sure the battle was epic, and the thought of him was cool, but his design was too lame. So Nelo Angelo. Again he's almost the same size, he's a black/dark knight which is my favourite theme for a bad guy/rival, he's a swordsman AND really challenging to beat. He even taunts us. I liked his design, his sword, the way he moved, the sound of his armor and almost everything, besides the fact he didn't say a word in the whole game.
But his laugh was ok to counter that.
Let's wait for the DMC4 boss. I really want to fight Dante with Nero^^