What is best use for RP?

No.. skins?

honestly, use your RP for skins since chests and skins can be received for free just from playing the game. Unless there's a champ you 100% know you want then I'd just say use IP for champs and play the champ and get an S for chests

I recommend using RP on anything that's not a champion, crate/key, or boost that you want to get. Basically, anything that can be purchased with IP you should purchase with IP (assuming you actually want it, that is), instead of RP. Boosts are useless (XP boosts are garbage for anyone who can buy them, and useless for us, obviously. IP boosts are wastes of money, because FwoTD's boost is better, and you can simply get more IP by playing more), and spending RP on a champion, when you'll get plenty of champion shards and IP from just playing is stupid. Crates/keys are up to you, but I'd recommend against it, unless you have TONS of one, and none of the other, and you're just sick of having 9.23*10^8675309 keys.