I'm completely uninterested in 4K nor do I expect that to ever change. At a standard viewing distance of 8 feet, you'd need at least a 65" TV to even begin to see a difference.
Chart: 1080P Does Matter - CarltonBale.com
The LotR trilogy is a favorite of mine. I had the extended versions on DVD back when they were first out, but now have the theatrical versions on Blu-Ray, which I picked up as soon as they were released. I probably should have been more patient and waited for the extended versions. Ah well.
I saw the first Hobbit in the theater and nearly fell asleep. I'm passing on the 2nd and 3rd. I'm just really annoyed at how they decided to milk it for 3 movies when 2 would have been sufficient.
Eh, anywho... well we just finished up watching Jurassic Park trilogy, which still holds up pretty well. I just got the Blade trilogy for Christmas... planning to watch that in the next couple weeks.
My favorites on Blu-Ray are:
Pacific Rim - man oh man I love this movie! Sure, it has some flaws, but they literally don't matter. It's giant mechas vs giant monsters! I grew up watching late night monster movies and loved them. This is the kind of movie I've been waiting all my life to get made, and it looks and sounds GREAT!
The Fifth Element - one of my all-time favorites. One of only 2 movies I've seen 3 times in the theater. I just couldn't get enough of it.
Empire of the Sun - one of Spielberg's best movies, yet so few people know about it. The Blu-Ray edition is stunning.
The Last Emperor - great movie about the last emperor of China.
Alien quadrilogy - I've gotten through the first 2 and it's like watching them again for the first time. Both movies still hold up really really well. For Aliens in particular, the director's cut is the one to watch, no question of it.
Terminator 1 and 2 - The original release of Terminator on Blu-Ray was garbage and looked awful. I recently got the remastered version for my birthday and WOW! It was like watching it again for the first time. Highly recommended!!!
Avatar - a great movie overall, even if the story isn't terribly original.
Avengers - one of the best action movies ever made. Seriously, as a comic book adaptation, it doesn't get any better than this. It does help to be familiar with the other Marvel movies preceeding it.
Hulk - the Ang Lee movie that people either really like or really hate. I love it. He broke from formula and directed it more like a comic book, I guess is how I can best put it.
Immortals - Visually a very cool movie, but a pretty weak movie otherwise. I love it just for the great lines delivered by Mickey Rourke.
The Iron Giant - still not available on Blu-Ray, but is available in HD off iTunes. A fantastic movie that teaches an important lesson about being true to yourself. I cried at the end of the movie in the theater, and still do.
Other random favorites... Life of Pi; Kill Bill; anything by Pixar; Sin City (freaking GREAT movie!); Sky Captain and the World of Tomorrow; Total Recall (the remake... visually extremely awesome and truer to the book, from what I understand); Lockout