Combos don't need to be infinite, as long as they kill.
The most lethal one at present is the Donate + Illusions of Grandeur combo. It costs a total of seven mana (not even in the same turn) and two cards to play, and both cards are blue meaning you can take advantage of a lot of library manipulation cards plus Force of Will and Misdirection.
Since the combo is completely blue, you can even add another color without losing too much, and in this case, you can go with black for even more tutors, Duress, and the other "most lethal" combo in Magic: Dark Ritual and Necropotence.
One fun thing to stick infinite combos into is a RecSur deck, though Great Whale recursion doesn't work anymore.
The Enduring Renewal + Goblin Bombardment + Shield Sphere works in a RecSur that uses Academy Rector.
Without Rectors, a pure creature combo is Living Death + Anarchist + Blood Vassal + Cathodion or Su-Chi + any damage dealer from Mogg Fanatic to Triskelion to Ghitu Slinger + Phyrxian Plaguelord or Fallen Angel. You might need to include something to search for Living Death that can itself be searched for by Survival of the Fittest (only thing I thought of was Noble Benefactor).
There is also another infinite mana combo that uses Workhorse, but it requires that you stick in Overgrowth and Earthcraft. I believe it was tried by Team CMU, and it also used Corpse Dance and Mogg Fanatic with two Overgrowths on a Swamp if the opponent tapped out.
the classic:
time vault/animate artifact/instill energy = infinite turns
not so classic:
time vault/voltaic key = infinite turns
w/ errata
time vault/GIANT FAN = infinite turns
as for mana:
earthcraft is a nifty engine.
earthcraft/fertile ground or wild growth or any land enchanting mana booster/horshoe crab or morphling or any 1 mana untapper = infinite mana
Mana Vault + Power Artifact does NOT work because you only get to untap Mana Vault once per upkeep. You'll have to use Grim Monolith and 1 Tinker + 4 Transmute Artifact to do the fetching.