I would definitely be Evelynn being all sneaky and invisible but I could also be Shaco. They're both fun and amusing champs especially Shaco , I could clone myself.
The only skin that i enjoy owning is Tyrant Swain since he's one of the champs I play most. However, the new Omege Squad Teemo skin is pretty good but expensive as it isn't on sale.
My favourite champion would be Swain/ Rek'sai. Swain is my mid main and he's so underplayed although he has such great potential especially post 6. Rek'sai is my main jungler who is just amazing when you get the hang of him. He's also very good at invading enemy jungle with his tunnels to escape.
I think this mode is great for the community. It brings a release of tension from ranked and normals to have some fun with team mates or friends. You could be losing horribly and feeding and still enjoy everything without flaming from team mates.