Pushing the limit ?
Futari Ecchi is so ecchi that a little more and you could label it hentai
Other than that...
Girls Bravo (both seasons) ...be sure to get the uncensored version....the censored one sucks beyond all reason.
Green Green is pretty out there, and it is so funny i had tears in my eyes from laughing.
Mahoromatic (both seasons)
Amaenaide yo! (both seasons)
DearS
Dokuro-chan (you will be in stitches from laughing)
This Ugly Yet Beautiful World is fairly ecchi as well.
Haven't seen Ikkitousen (only a bit), but it seems they have set out to test how many panty shots it is possible to cram in to 26 minutes (or w/e the ep length), and then add some more on the top