Well, if you wan't a lot of people o would suggest the hub in shibuya.
Right when you walk from the crossing inside the walking Street.
In Front of the gas panic ( also worth a look if you're in the right mood to dance and be a bit naughty).
This is the most crowded one in shibuya the other ones are decent AS well though.
This particiular one though, has lot's of people on friday and saturday until the last train.
You should also try the bar all the way down the Street, almost by the outback.
It serves beer for 300 Yen and the ratio sucks most of the time. But i've been able to pull some with a friend to different bars and on the next day in the bed.
So it's always everywhere possible if you push the right buttons.
I've been to the hubs in ikebukuro as well, mostly with a Group of foreign guys and japanese girls. The ratio was always good, they still meet every saturday starting from around 5:30. but i as well forgot the Name of the hub.
It's close to a pachinko spot and a family Mart though.
After some time sometimes the hub gets a DJ and things getting pretty good.
Good luck and try your thing.