what do you say to commend a teammate's good play?

"Uninstall". They love that.

i just say "nice" or "gj"

Depends on the play, imo.

Assuming I say anything: "nice" "gj"

Anything more than that takes time and focus away from actually playing the game in my opinion.

I keep it simple. Always type a quick ''gj''

I do the usual gj or nice. But occasionally I like to hit them with calling them a smurf

nice naice noice naicu

Usually just GJ and the 1st name of their champ eg Gj G = Good Job Garen.

Not enough time to type more.