I wouldn't recommend Battle of Algiers for watchability. I saw it at the movies and it is an amazing but terribly upsetting movie. I bought the Criterion about four years ago and still haven't watched the DVD.

It depends on what you like, doesn't it? For some it's Seven Samurai, for others, Salo.

But my 2 cents, most rewatchable is the first I ever got, Beastie Boys Criterion Collection.

You can start at anytime any song, listen to songs in original, remixed, acappella form, multi-angle comparisons, making ofs, commentaries, 5.1 or 2.0, it really showed the versatility for DVD for me.

by Brakhage is the Criterion I've popped into my players most often.

Criterions that I have rewatched the most times:
Seven Samurai
The Rock
Chasing Amy
Yojimbo
Armageddon
The Killer
Hard Boiled

My most watchable Criterions:
Le Samouai
Army of Shadows
Two-Lane Blacktop
400 Blows
Dazed and Confused
The Lady Vanishes
The Life Aquatic

A Canterbury Tale
The Lady Vanishes

Sullivan's Travels
Rushmore
The Blob
The Third Man
Rebecca
Dazed and Confused
This is Spinal Tap!

Another for The Last Temptation of Christ... it's the only Criterion title that I own on laserdisc that I upgraded to dvd. (Criterion version, that is)

Easy: Spinal Tap

Fear & Loathing in Las Vegas
Yojimbo (ft. an excellent commentary by Stephen Prince)
Spirit of the Beehive
Brazil