So far I've won all of my One for All modes that I've played, whether due to luck or stupidity on the enemy's part.
First match was Kalista vs Soraka. The enemy Sorakas couldn't hit anything jumping around that much, and I had a point where I was soloing their team, managed to kill 2 of them before dying myself. Ended 17-1-7.
Next match was Lulu vs Gangplank. We had an even spread of 3AD, 2AP Lulu, and while it was a rough start, they all ended tasting purple. Ended 12-2-14 with a more attack speed inclined build.
Then there was the Jayce vs Zilean round. They were annoying, as anyone who didn't want to die basically didn't die. But when 2-3 Jayces chain each other gates, it's a constant barrage of burst damage, and none of them had the forethought to get Zhonyas.
Last I had a Mordekaiser vs Yasuo, which went better than expected. Another rough start, but they had a jungler, and it was basically playing a game of Pokemon. 2-3 of them ulting 1-2 of us left them wide open to just get smashed and turned into minions. 5v5 became 6v4, 7v3, and so on.
I just wish they'd have these fun game modes more often, or simply integrated them like DotA 2 has, with separate patching. And I really want to play Matroshka Kench. It'll be like a horror movie/nature documentary. "Here we see the catfish, giving birth to it's young... which then also gave birth to another, full size specimen, and oh Christ they're eating everything."
There are a few characters that should do extremely well in this mode, particularly due to their ability to go alternate builds. Well, technically anyone can, but a decent spread of AP/AD is always nice, like with Mundo, Tahm Kench, Kayle, Lulu, etc.