Rosario to Vampire is about the extent of what I will be ok with as far as me personally. I think Gurren Lagaan did it pretty much perfectly though, although Code Geass I think does well too.

I do enjoy a certain amount of fanservice, but there are those series that just take it too far. For example, I love Oh!Great's art, he is truly a master, and he does know how to draw a very attractive (if unrealistic) feminine frame, but the amount of fanservice and, honestly, straight-up nudity in, say, Tenjo Tenge WHERE IT SERVES NO PURPOSE was ridonculous. Same thing goes for Air Gear. History's Strongest Disciple Kenichi likes to push it as well, but it's not as aggressive about it, for some reason.
 
I'd have to agree with DJTyrant that Tengen Toppa Gurren Lagann did it really well. I guess I'm ok with it so long as it does not become a goal unto itself that harms your enjoyment of the plot/atmosphere.

Fan service isn't good mostly when your in a dramatic moment or like a monologue