If you have a low br then play 6,9 or 18 max SnG. But everything depends on what do you like to play most... I started playing 9,27,90 SnG,then switched to the MTT's.
i prefer playing on fulltilt
the micro 0.25 and .5 single/multi table both give a decent chance to increase bankroll. i have moneyed in 3/7 micro 0.25. and 0.5 single table games are quite easy too.
Just play the one you enjoy and/or are most profitable in. If you don't know which then test out the waters and find which you enjoy most and then do some grinding to find out if you can be profitable.
If you want to narrow it down then a big factor would be time, if you play shorter sessions 1-2 hours then you won't want to play much beyond a 45 man sng of regular speed but could sometimes fit a 180 man turbo speed within that time period.