I would vouch for Ascension, TT Hexakill, and Definitely not dominion. pretty much because they are far more unique, compared to the core game. Ascension and Dominion, both have greatly different objectives than SR, making them a much more unique experience. And to a lesser extend, Hexakill, simply because the game dynamic changes when you put 6v6 on a map built for 3v3.
Urf in my opinion, is interesting, but it doesn't really change the game dynamic significantly, (at least in a sense where things feel fresh.) Though I bet we'll see it again, maybe after the next new champion, or next major update. It might also be the reason why we haven't seen nemesis draft, even when that game mode should be easy to implement.
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Bring back the pirate mode on summoners rift!!!!
Something tells me that Riot is going to want to get a new set of items before bringing back Black market Brawlers. And that could take a while, since the Featured game mode team is still focused on this up coming nexus siege.