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My reply may not be what the OP wants, but anyways...
Inception vs Memento
I'm not a fan of either, but been meaning to rewatch Memento at some point. So, if I'd have to choose either one at the moment, I'd go with Memento.
Fight Club vs Zodiac
I liked Zodiac and hated Fight Club. So, despite the fact that Fight Club probably deserves a rewatch I'd definitely go with Zodiac.
The Terminator vs Blade Runner
I love both so it would be a day-to-day decision. I did have Blade Runner higher on my top-25, so for the sake of this "poll", I go with it.
Goodfellas vs The Godfather
I rewatched The Godfather quite recently and liked it a lot. I'd remember Goodfellas to be great as well, so I think it would its turn to get a rewatch. No idea which is better, at the moment.
Star Wars vs 2001: A Space Odyssey
I've mentioned this a few times, but I've never managed to watch 2001 in one go. I just can't get into it. Star Wars, on the other hand, is great.