I really enjoyed PlanetES, because I've always been interested in space and space travel, and it really presents one of the most believable scenarios for the future. The series gets off to a slow start, but really picks up. The whole series is mostly just character development, but it does a great job at it. If you've always liked space (real space, not space opera anime) you'll like it. It's really an anime you'll either like or hate depending on your interests.

One Piece takes a huge commitment. (Following is on the sub, the 4kids hacked apart versions are unquestionably unwatchable). I downloaded the first 20 eps or so, but I realized that I just wouldn't be able to keep with it. It never really caught me, but from what I understand its really good. If you can commit yourself to watching it, you'll like it. Everyone I know who watches it is a fanatic about it. All it takes is that one push.

Hajime no Ippo is exactly the same.

The FMP series' are highly recommended, the perfect balance of comedy and action when you watch all of them.

GunXSword is a totally forgettable series. I'd say don't waste your time with that one.