who to one trick?

Singed top Brand support Rammus jungle for low elo. Low elo includes plat.

Edit: I am curious who down voted and why? I was fooling around for half of the season and dropped to Silver 4 twice as a result. Then I played Singed and Brand to climb from Silver to Platinum. Then I played by myself in Flex queue as Rammus to get to Platinum. My recent match history will show you that.

Of course... I climbed from Silver to Gold 3 in Flex queue while playing Attack Speed + Crit Support Bard. Then went from Gold 3 to Platinum from the jungle role.

You should one trick a champion you like, not just one that's "good". Play every champion until you find one that just clicks with you. For me that was Taliyah, and I had a pretty high winrate with her even when champion.gg had her at like a 42% winrate.