Well, other than the two enrage fights (Belial and Butcher) on MP5, the toughest fight I ever won was a little adventure in MP5, Act II. I told this story once in your What MP Level Do You Think I can Handle Thread, but it makes me feel tough and superior so I'll tell it again here. Like Vrkhyz, I play self found, so adjust your expectations accordingly.
Booted up the game, took the Waypoint to Black Canyon, and headed left. Immediately picked up an elite fallen pack. I circled around the rock formation there, and what do you know, there's a yellow pack of overseers. Okay, I think to myself, I can handle this. Just move when the overseers jump, kite the two packs around for a bit, save serenity and SSS for when you're in trouble . . . this is totally doable. One of the two packs had plague, and the ground was getting awfully green, so I kited them east toward that building that's just to the right of the waypoint. Roll my eyes when I see another fallen pack and some wasps join the fray. I'm actually happy to have some white mobs running around . . . free spirit and LOH. So I start kiting the mob around the building, and . . . another champ pack of overseers. Sigh.
So anyway, kite, kite, kite. I die a couple of times when dashing strike gets caught on the building (not the dash into the phantom zone bug, just the one where a small piece of scenery stops you from getting where you're aiming). But things are going pretty smoothly, I'm whittling the three pack down at a decent rate, spirit is staying up, LOH is doing it's job, serenity and SSS are juggling nicely. And finally I kill a blue with exploding palm on it and the group is down to 3/4 its original size.
So I kite them around the building a few more times until I kill something else with exploding palm on it, wiping out the rest of the stragglers. Start picking up loot, and look down at my nephalem stacks.
*blink* Turns out that group of fallen I picked up when I first moved over to the building was also a blue pack. So yeah, I just survived a MP5 4-pull with two deaths, both caused my dashing strike not playing nice.