I don't agree that it is the best SF movie ever, but you're quite correct about one thing: the cult classic "A Boy and His Dog" has the best ending in MOVIE HISTORY! It's very stupid (in places) and very low-budget, but the ending is worth the price of admission (or, in this case, rental).
It's sad but true that the big SPECIAL EFFECTS flicks they make now-a-days have none of the class or creativity of films from the 50's, 60's & 70's. I will give Spielberg credit for single-handidly trying to revive the genre with AI, Minority Report, and others, but so far he hasn't come up with a knock-out...
The only film from modern times that compares with the oldies is "The Matrix" --but that's more of a Kung-Fu film than SF...
I would recommend a movie called Dark City, from maybe six years ago. It is like The Matrix without the kung fu. Really excellent flick.
Did you see I,Robot with Will Smith?
50's Classics:
- 'Them'
- 'Tarantula!'
- 'The Flying Mantis'
- 'The Thing'
80's
- 'Liquid Sky'
- 'The Road Warrior'
90's
- 'Mars Invades!'
00's
- 'Galaxy Quest'
IMHO...