While I fully agree that Gladiator (all the way around - movie, plot, cinematography, acting, you name it...as well as the extra "cha-ching!" disc with all its own extras) is the best hands down, I have to give Titan A.E. its due.
I purchased Titan A.E. this weekend and was happily impressed. Reminded me a lot of the original movie Heavy Metal. And no, as of yet, I have not checked out Heavy Metal 2000...my understanding on that one from various posters here is that HM2000 is $hit.
Another good extras DVD, and a sweet movie in its own right for any action buffs is Nick Cage in Gone in 60 Seconds. Pretty ridiculous amount of extras there...and good too! Loved the \'stang jumping the bridge traffic jam. Way too cool.