First off, I’m gonna say Eureka 7 is a show that gets better as it goes on. Also, I saw that someone was recommending Brave Bang Bravern which is a favorite of mine. If you want something a little more classic though definitely check out Gunbuster (also known as Aim For the Top). It’s only 6 episodes and it’s some of the coolest stuff I’ve ever seen. Really good depiction of time dilation.