I voted Mai-Otome, because it's just too much fun. Presuming that you liked Mai-HiME enough to buy Mai-Otome, you should get an extra kick out of it even though they're very different series in tone.
Banner of the Stars is also a classic, again presuming that you've seen and liked Crest of the Stars there would be nothing stopping me recommending that.
ROD is a great action series, really good animation, pretty intriguing story once it gets going. Also it's by the *awesome* J.C. Staff who Fabby apparently thinks suck, but has now recommended two of their series in short order.
Baccano! is truly magnificently realized, very strong characters, very strong atmosphere, another one I wouldn't put you off.
I don't like the TenTen or Air Gear anime, because they suffer so badly compared with the manga. The Air Gear anime in particular was poor imo.
Terra He I haven't seen, or read, but is also on my list of things to check out some day. It was written in a time when women didn't really write sci-fi, so even just from that point of view it has some interest.
Mars Daybreak and Elemental Gelade I thought were both pretty bland, but it's a few years since I saw anything of either of them.
Rozen Maiden: Träumend, none of the new dolls are quite as good as the ones from the first series, but as with the other sequels it's definitely one to watch if you enjoyed the first season. It's much more plot driven, too.