Played 3 games so far, Evelynn first - was interesting - 0.3 second Q spam was kind of terrifying - half the scariness of my ganks was just the terrifying gnashing of spikes, as though the ground itself were chewing them to death. She fell off late game though because the enemy team had so much hard CC for me (Malzahar ult was the worstttt). I didn't read the rules though before the game, so I had my Magus jungle item with 20% CDR first, that was a mistake ><
I think if I played it differently Eve could have some potential in URF (maybe AD Eve with 1.8 second Ravages?).
Second game I went Morgana because I liked the root spam + black shield spam + ult always up + 3 pools active at a time (spell vamp doesn't hurt either in this mode!). Top damage and kills even though I went in as support, she's clearly good at this mode - but we ended up losing (respawn timers get soooo long). Speaking of which, can we Revive in it? That'd be OP!
Last game I played AP Ezreal mid - combining his high AP burst with 16 second cooldown AP ultimate. Once we hit six mid-lane was unwinnable for them, even if I just threw every other wave down the lane - that'd wipe out the next 2-3 enemy creep waves - and I could still throw the off waves across the map as needed. Won that game pretty handily despite an enemy Zed being pretty broken (spams ghosts and his Q poke every like 2 seconds - absent the energy regen that normally tethers him regardless of build/level).
I haven't seen a Jinx yet, but I suspect she would be very strong - without mana she could rocket poke forever from L1, and her ultimate must be similarly ~12 seconds at max level or something. Nocturne/Pantheon were my other guesses for strong champions - frequent ults - and terror/stun spam. We had a pretty strong Fizz player that was hyper-hyper-mobile/annoying, so I'd recommend him if you're good with him.
Lastly, despite seeing a LeBlanc in every game (and understanding that people want to teleport spam) I have yet to see anyone do well with her - her combos are already so fast without URF that I suspect they just can't pull them off - and she's too frail for the prolonged battles this mode tends toward. I haven't seen a Kassadin yet, but I suspect he would be way better - shield spam, wave spam, can spam teleports the entire game (would it be every 0.4 seconds?, 80% CDR on an already only 2 second CD?).