I think MK had his opinions on this, I'll try to fish it out.
EDIT: It was warrior, and his list went:
Talos.
The HoonDing.
Trinimac.
Vivec.
Leki.
Reman.
Auri-El.
Wulfharth.
Morihaus.
Pelinal.
That's my list, and pretty much in that order. Though Vivec did kill Tiber Septim once...but I mentioned Talos, not the Emperor.
Also, I'm going to assume Vivec, pre-CHIM (assuming V-man did achieve it).
For what I'd say, Alduin. Alduin eventually does consumes all (save for the stuff MD and Shor saved from previous kalpas), and not because we saw an avatar of him at the end of OB.
If this is about martial ability, I'm going to have to disagree with MK and say HoonDing.
Also, there is a problem with this list; a lot of the beings mentioned are not even at the level of the gods, so by default, they're weaker (Pelinal, Morihaus, Shezzarines), or have already been defeated/nearly killed, even in their hay-day (Alamanexia, Sotha Sil, Dagoth Ur, Wulfhart, Trinimac).
Also, MD should be given more credit, he really is the only one who has been able to cross and cause a lot of crap on Nirn than any of the other princes. Hell, Alamanexia and Sotha Sil, together, just barely defeated MD while his avatar was on Nirn at the end of the 1st era. Hell, I'd say they wouldn't have been able to defeat him if they didn't tap into the heart and gang-up on him.