
Fucking duh, man. 
Gainax mecha shows in general, really. In fact, I find that you can appreciate them a lot more if you've actually seen Gunbuster, Evangelion
and Gurren Lagann. It's like three different outcomes to a similar beginning. Mind you, the only real similarities the three all have are the starting condition of the protaganist, so.
Ahem. Basically, you should all watch Gunbuster, for it is made of awesomely awesome awesomeness. >.> <.< Eva, on the other hand, is... I don't wanna say "love it or hate it", as I know a few people who were neutral towards it, but... It's easily one of the most controversial anime out there, and the latter half/the movie are incredibly fascinating to watch if you happen to find things like psychology, philosophy, theology et all interesting (a love of surrealism and subversiveness also helps >.>).
Of course, Gunbuster, being... Less popular than it should be (;_;) tends to avoid the one thing about the other two that... Annoy me somewhat: Fandumb. >.> Seriously, people just like to flanderize the characters from Eva and Gurren Lagann until all meaning is stripped from their characterization (Shinji and Kamina get it the worst, although in completely opposite ways). Sigh. But of course, that's hardly enough to ruin the appeal of the shows, so... You should all watch them. Especially Gunbuster. Not enough people watch Gunbuster, and it would make me smile if more people did.
...Please~?
...My other big pick would be Excel Saga, but... If I'm going to recommend that, I'd like a little time to clear my thoughts out so I can make it clear exactly why you should be watching it. >.> <.<