Knights of Sidonia has great visuals and lore ideas but boring storytelling and characters.
Argento Soma had interesting aliens and story but the robots were pretty basic looking.
Bokurano has great visuals and very interesting story, but it has some heavy topics. (Epstein related...) Also I think the manga is better, but the intro to the anime is worth watching just for the song.
Fullmetal Panic has great robot battles, but you might not like the combination of highschool sillyness and serious war story.
Gankutsuou has wonderful visuals and only one Mecha battle in it, but that qualifies it ;). It is an anime adaption of Count Monte Christo
Kenran Butou Sai: The Mars Daybreak has a nice combination of adventure setting and Mecha fights.