I always say slowly with a dead pan face "Tremendous", really emphasizing the word but not smiling at all.
I also like that previous poster's suggestion of "Outstanding" as well. I think if you can create a dichotomy between the power of the word you use, but having a serious expression when saying it, without any fake emotion in it, it lights up the chick's broca's region and triggers fascination in her.
It also creates the impression that high states of well-being are pretty much normal for you, and no big deal.
I don't think that manic happiness impresses potential pick ups, unless everyone is feeling the same way and its kind of like a group vibe that everyone partakes of. But on the other hand I don't think a standard response like "not bad" or "pretty good" does anything to ramp up her attraction response either.
Perhaps the word "Superb" would work as well, the key is to say it slowly with a low, smooth voice, while making intense, smouldering eye contact with her.