Spider-man 2 is the best superhero movie of all time, period. No room for discussion. :D

Anything by Quentin Tarantino, most notably Kill Bill 1 & 2, Pulp Fiction, Reservoir Dogs and Inglorious Basterds. The old Disney movies are also always great to rewatch, the original Star Wars trilogy, the first three Indiana Jones. I also have a soft spot for Ms. Doubtfire and The Mask.

Back to the Future trilogy
The Godfather
The Shining
The Prestige
Forrest Gump
Hot Tub Time Machine

This is Spinal Tap!

edit. Ok ok I admit I love watching Along Came Polly and Holiday over and over again too..

My list is going to be a bit of a weird one for most.
1. Rear Window
2. Vertigo*
3. Casablanca
4. Clerks & Chasing Amy
5. American Psycho
6. Commando
7. Breakfast at Tiffany's
8. His Girl Friday
9. Forgetting Sarah Marshall, Naked Gun 1 and 2, Bridesmaids, American Wedding**
10. Insidious & The Conjuring 1 and 2, The Thing, Dawn of the Dead (kind of hard, I usually watch Horror films in like groups of 10-15 over a week once or twice a year, and these ones have been among them each time for the past few years)

These aren't necessarily my favourite films, but the ones I tend to rewatch the most. I grouped up a few because I almost always watch them together.
* Vertigo is by far the film I watch the most on my own. I pretty much only rewatch the others with wife and/or family.
** I used to watch the full trilogy, but have mostly just been watching American Wedding in the last decade or so, I'm not saying it's better than the first two, just that it's super rewatchable.

But who am I kidding! The most rewatchable film of all time is CLEARLY this one:

For animated films:
1. Kiki's Devilry Service
2. The Secret of Nimh
3. Fantastic Mr. Fox
4. Nausicaa of the Valley of the Wind
5. Fantasia
6. Frozen (the first one, not the second)
7. Tangled
8. 101 Dalmatians
9. Mulan
10. Akira

Note, I like Princess Monoke the best of the Miyazaki films, but find it exhausting to watch compared to some others. Same goes with Satoshi Kon films (despite having Vertigo by Hitchcock ranked so high on my list, which is his chief influence); Satoshi Kon is my favourite anime creator of all time.

Those are all really good films. Although I find Goodfellas and the Shining a little exhausting to watch (somehow not the Lord of the Rings, despite it being LONGER) even though they're both among my favourite films of all time. I did used to watch Goodfellas a lot, that and Silence of the Lambs were among my most watched films of the 90s.

These two are disgustingly rewatchable.

The Princess Bride is also great, the whole list is great.

Predator and Aliens are those two action flicks that are always way more awesome than I remember them being each time I rewatch them.

I forgot about those two! Excellent flicks, and very re-watchable. I'd say Titanic remains to this day the quintessential romantic tragedy in film. Maybe even the best romantic film of all time barring Casablanca.

Aliens
The Thing (1982)
Predator
Starship Troppers
Lord of the Rings
Friday
Die Hard 1, 2, 3
Vampire Hunter D Bloodlust
Police Story Films

Casablanca

This is one of those films that every person should watch at least 10 times. It's endlessly entertaining and filled with iconic scenes and lines that make it truly a unique masterpiece among all Hollywood films. Some others have this on their list, there's not much more to say, it came out in 1942, in the very heart of WW2 the year before the tide had turned with the development of anti-U-boat weaponry and Nazi military disasters in the first half of 1943. The fact that the film is full of iconic scenes is one thing, but when you begin spotting what emotion is acting and what emotion is real, then it takes on a whole new meaning.

Why is it rewatchable? All of the iconic scenes and lines, and the entertaining and emotional scenes.

Rear Window

Rear Window is, perhaps, the greatest concept film ever made. One of Alfred Hitchcock's most popular and critically acclaimed works, it features Jimmy Stewart, who plays Jeff, a photographer with a broken leg, and is slowly going mad of boredom and alcohol consumption. To make things worse, a summer heatwave. Also better, because his neighbours across the yard all have their windows open to cool off, and he spies on them, and eventually brings his nurse and his girlfriend Lisa (played by Princess Grace) into his less than moral behaviour. There are a series of multiple stories going on, throughout the neighbourhood, and stories inside the apartment (where the camera never leaves). Then Jeff believes he sees something happen: a murder. He builds the case up in his head, but is it real? This, in my opinion, is one of the most entertaining and rewatchable films of all time.

Why is it rewatchable? There are so many little things going on in this film, and it is, a concept that inspires the imagination, and the entertainment value.

Breathless

This film unifies the two previous ones. Alfred Hitchcock broke the rules of cinema, and was among the first true auteurs. Films like Breathless became the first step inspired legacy of Hitchcock, which, when examined a whole, shows the profound impact he had on the film industry as a whole. Breathless follows an anti-hero murderer who is heavily inspired by Humphrey Bogart. It takes a very non-traditional view of relationships, but perhaps one that is more realistic except for one element the protagonist hides from everyone he comes across... he's a psychopathic murderer. 

Why is it rewatchable? The film is full of style and substance, and while heavily influenced by Hitchcock, it's also one of the most unique films of its time.

American Psycho

And while on the topic of psychopaths, another film in the same legacy.

American Psycho is a dark comedy that satirizes the shallow and selfish psychopathic nature of Yuppie culture. Some people latch onto the mystery of whether or not Patrick Bateman is a killer... this element in the author Bret Easton Ellis's opinion is one of the reasons why he didn't feel American Psycho was a good fit for a film adaptation; because a film needs a certain structure and level of clarity and answers, while books can be more free and abstract. In the book, it's not whether or not he killed the people that matters, it's the dark comedic element of a guy killing (or imagining he kills) people for shallow reasons. It's difficult to explain why, but in the book, while the answer is even more vague than in the film, it's not something that strikes the reader as important in the way it does in the film. The book didn't have a plot structure either, it was purely scenes satirizing the New York yuppies of the mid-80s. The film is remarkable in that the material is around 95-98% from the book, but somehow has a completely different feeling. They took the order of information in the book and re-organized it into a plot. For my money, the book and the film are both great. The film is full of some of the greatest dark comedy you can find in film (and the book is similarly full of some of the greatest in books).

Why is it rewatchable? Every scene is just so bizarrely humourous in how shallow these guys are, like the below one where the fact that a guy has a slightly better business card sends the main character Patrick Bateman into an anxiety attack. The fact that he managed to get a reservation at an elite restaurant on a Friday is what REALLY sends him over the edge.

There's only one logical solution for Patrick Bateman in order to recover from the fact that a guy had a better business card and was able to get a reservation at place on a Friday where he was not.

Reviewing Huey Lewis and the News before brutally slaughtering him.

Another honourable mention, from Hitchcock's library: Strangers on a Train

The setup for the whole film is in this 2 minute clip, and again, this one is crazy entertaining and has some great acting. I am surprised they didn't remake this one with Edward Norton and Ben Affleck, it would have been PERFECT. It's probably not too late for it.

And I have to at least give a shoutout to Hitchcock's North By Northwest. If you like Bond films, this film almost certainly had a gigantic influence on Dr. No through On Her Majesty's Secret Service. While Cary Grant isn't a government spy, his role in the film becomes very similar to that of Bond. And if you are a fan of From Russia with Love, On Her Majesty's Secret Service, and even Casino Royale, then this one will be a great watch for you. Cary Grant and Eva Marie Saint both beat any Bond and any Bond woman, IMO. This is like a Bond film with extra charm and extra plot layers. It's a fun film worth many watches.

Anyway, as far as actors go, you can't go wrong with Cary Grant, Humphrey Bogart, or Jimmy Stewart. Their films are nearly all highly rewatchable, IMO.