katarina is really easy, snowball hard and noobs dont know how to play against her/cc her
blitz jinx works really well with good synergy since u can put chompers behind them and its really easy for blitz to land a hook
Lee sin is a great pick. Go hydra last whisper, great damage can split well get super fed early and close out games. The ideal game for peak xp is a 20 minute 5v5. If your 5v5's are going to 40 mins however then bot games become a better alternative.
Source: Recently leveled a smurf and did a lot of research