Some I went through.
Aeon Flux is garbage imo, watch the original animation if you are eager about it. That was too weird for it's own sake, but quite unique. Body snatchers is so-so, watchable, nothing spectacular.
One thing Tron can be called is over-rated, not under-rated. It's one of the most beloved movies of 80ies, not every movie of its caliber has this large fan base, I like it also

.
Sleeper should have been real good, but Allen hasn't flourished to his full bloom since then, a coupla laughs and ideas to catch there though.
Existenz is basically on the same floor with 13th floor and Matrix. But it's actually a rehash of Cronenberg's own Videodrome this time on virtual reality console. It gets mediocre most of the time, the characters are cartoony, that's why it sank I guess. Quite watchable.
Rest I didn't watch, except Gattaca. I saw a Boy and His Dog praised also, might be worth a look.
The Man From Earth from last year would be one the neatest addition to your list. Can't say it's underrated, for a movie extremely low budget and cinematically not much involving it got quite a well reception by many. In a few years it'll quickly turn into cult.