Theory: Picking Fill will increase your chances of winning the match
Assuming you can play all roles efficiently and assuming you're comfortable with any role.
When you pick fill, you're filling the gap for your team, that means the players on your team will usually get the roles they're comfortable with and will play better.
Most of the time you will get support, but some times you can get top or jungle, I never got adc or mid playing this way. Just pick the most OP shit on your role and you're set.
When I get support I pick
, why? Because she is too easy and OP at the same time, even a brainless monkey can play her effectively. You can play Janna with a blindfold and still win lane & game... She can prevent even the dumbest of adcs from dying in lane and can make good ones win trades hard with her shield.
Top lane I get Tank
. You can't lose lane with Tank Akali because of all the sustain you have. You can easily farm under tower with your no CD E to help last hitting. After sunfire cape you just faceroll your opponent and splitpush all game.
Just go here: http://champion.gg/ And pick one of the top 5 champions on the role you got, you don't have to play perfectly, just basic game knowledge is enough to win playing this way.