I guess it's the enraged Skeletron Prime, it runs faster than UFO, almost one-hit-kill and has higher health than Dungeon Guarian.
But for the Poll, I think the frost moon. With UFO, the Dungeon Guarian is very easy, but even with end-game gears, it's still difficult to get 20th Wave (especially in normal mode, as the score of the mobs is halved).
(by the way, before 1.3, I even tried cheating with locked health but still rarely reach the final wave, only managed to do so a few times with the Horsman's Blade and lost's of mechnism, not even Coin Gun with Platinum Coin)