Primer's been on my list for a long time but I still haven't gotten around to seeing it.

Hot Tub Time Machine I liked, except that it has the same problem too many time travel movies do: they say exactly what they need to do and follow that formula.  There's little chance for surprise or going an alternate route.  It's either "they'll do this or they won't do that."  Ad wayyy too much of the movie was in the trailer.

Though it is one of those movies that's growing on me like mold, I'll watch it all the time and it cracks me up more now than it did the first time I saw it.  Hoping that everyone comes back for the sequel.