If a player spams "gg ez" all during the game, end game, and after game, I'm reporting them.
I think it's very unsportsmanlike, and makes the losing team feel like dirt. Win with modesty I say!
Also I feel like Riot may agree with me because this morning I logged in and got the message saying someone received a penalty from my report.
The only thing I remember reporting lately was the gg ez spammer, though I could be wrong. The only other thing I report is leavers.