Mugendramon. He's the reason I actually like Digimon in the first place.
In every Digimon game I ever played, my main goal is always to make/create/hatch/fuse a Mugendramon.
I must say that so far, in every single one I've played that have a playable Mugendramon, I got to make one - with the exception of Digimon Battle Online, which it didn't had a playable Mugendramon during the time I was playing, and when I found out that they introduced that guy... it was too late.
DBO died a horrible death - alone, unloved and full of hackers. RIP in pepperoni fogazza marcone.
My favorite Mugendramon has to be my Digimon Masters one.
I played that crap for sometime and... it wasn't much fun. Everything was meh and the combat was a mess to begin with, but the best thing is that you can actually have a freaking giant Digimon by your side, and that was incredibly awesome.
Something something hatching eggs with a certain %. I don't remember nor care about it, but having that giant machine dragon walking around with me was a dream coming true.