Who do you hate facing in One for All?
For me personally it is Blitzcrank. Because not only is having to dodge 5 hooks constantly exhausting and unfun, but most of the blitz players on the blitz team are extremely toxic. Like it's not that impressive when all 5 of you use your q, and you hit one person and melt them. You're not "skilled," and we weren't "outplayed." It's supposed to be a fun mode.... don't ruin it when you know the tides are against the other team.
. They're really tanky and killing them is neigh impossible since they can all just devour each other and get away when they're low! You can forget about escaping them when you need to, though, because they have a million ways of following you and CCing you down.
*hits lvl 11*
and 
Can literally keep your entire team stunned forever and never die. Extremely annoying.