I'm looking for opinions on what are the best dungeon crawl mods out there (preferably challenging ones for high-level characters)

I've seen this list: http://www.mwmythicmods.com/dungeon.htm which is nice and exhaustive but unfortunately doesn't provide much in the way of feedback.

So does anyone have any dungeon mods they'd recommend? Also, has anyone tried those "500 level" and "1000 level dungeon mods"?

Many of the 500/1000 level dungeon mods are just genmod dungeons. You can make those yourself as well.

Have you played http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6068?

"The Horror Mod" is mostly a very large dungeon, with bosses and puzzles. It can get quite difficult.
Keep in mind that it is not Lore friendly, and some people report problems, although I've played it through twice with no problems.

"Annastia" has some neat, large dungeons. But they are geared more towards a level 15 character.

http://planetelderscrolls.gamespy.com/View.php?view=mods.detail&id=477. I made this for higher level characters (30+ to join the faction). Plenty of dungeon crawls.

http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=5865 I played the first one. This one is a bit on the larger side (500+ mb).