Hrm, I wrote a novel ... nostalgic much, Ravenface?
My five favorite zones would probably be a list of original and Kunark-era dungeons, but I've varied it up a little by picking the best examples of different kinds of zones from different eras. In general, I tend to like dungeon zones with a pretty consistent set of qualities: they need atmosphere, areas that feel distinct from one another and interesting, challenge that's *fun* instead of annoying (hello Velious dispel traps), and good itemization (loot!).
I didn't pick any outdoor zones because none particularly stand out, but I also *love* the outdoor EQ world, especially Antonica and Faydwer. Leveling up Gfay/CB with trips to Steamfont, Butcherblock and Kaladim (to turn in orc belts!) before finally venturing into Unrest for the first time ... the variety of the whole Nektulos/Neriak/Lavastorm/Commons/FP/Ro/Oasis area ... Rivervale, Misty Thicket, and the run across the Karanas to Qeynos or Lake Rathe ... exploring the Ocean of Tears ... all serious fun! Subsequent expansions had outdoor charm, too, of course, but while Kunark's gigantic, forested zones do a great impression of getting the undiscovered wilderness theme across, they're just too big and too empty to have the appeal of the original continents. Ditto Velious.
Anyways, favorite zones:
1) Sebilis / Lower Guk
Okay, I'm cheating, but these two high level frog houses played the same role in subsequent expansions for me. Seems like they're getting a lot of attention in this thread, and they deserve it! I loved the dungeons of EQ's first two expansions, and probably had more fun in them as an enchanter than anywhere else in game. There were a lot of good ones, but these two were the best. Lower Guk was unrivaled in original EQ (Sol B was okay, but not in the same league!), but Kunark does have a close runner up in Howling Stones.
I think my favorite areas in these zones were the Savant/Sage/Exe/Cav complex in Guk and Chef/Bartender wing in Seb, though 4d/the crypt sure had ambiance!
2) Castle Mistmoore
Any list of EQ's best zones would be incomplete without one of the original mid-level dungeons on it.
Mistmoore was flawed, and kind of evil. The level spread was too large, mobs were very dense in some areas of the castle, and there were geometry problems that could lead to killer trains from upstairs in the castle. A lot of people flat-out avoided the place as a death trap, but I loved the atmosphere and sense of place. The trains were legendary, and Bristlebane had an infamous high-level ranger tool named Boneman who would train half the castle out to the zone line periodically (although probably not on purpose).
The zone is a big castle full of dark elves who spawn as vampires at night! You start out in a manicured courtyard and work your way up through the graveyard (with secret passage into the scary back of the castle/tower area), the pits, and into the castle.
Inside, you've got everything a castle should have: ball room with werewolf gypsies, indoor garden/peristyle with cloaked dhampyre spawning at the fountain, kitchens (in the back, of course!) with tiny, cramped quarters for the named butler and maid behind them. Upstairs you find a library, a court room with Princess Cherista, baths with Lasna Cheroon, and, through some tunnels, an elevated tower with Garton Viswin.
Itemization is also an important part of drawing people to a dungeon and making it exciting, I think, and Mistmoore named dropped some very desirable stuff.
I thought about picking the original CT for this spot, which rocked and really taught people how to play well in groups, but Nydi already mentioned it. Sol A is also pretty good.
3) Chardok
I'm counting this as my favorite Velious zone, since it was revamped and itemized during SoV. Funny but not really surprising that Velious' best dungeon is actually a Kunark zone.
The itemized Chardok, like Mistmoore (and unlike SG, DN, and to a lesser degree Velks), had some really great swag in it, which I've already said I think is important for a dungeon. A group prepared to tackle the dangerous lower areas could come out of Chardok with some seriously nice stuff.
The sole respect in which Velious dungeons improved on Kunark dungeons was that most of them offered challenging, all blue-con camps/crawls for a full group of level 60's or high 50's. With areas like lower foreman/underboss, kennels, and bank, Chardok did it best, imo.
Velks gets honorable mention for the pit/castle areas. SG and DN don't make the cut - for single-group crawls, they just brought Chardok-style difficulty without the fun. DN in particular was full of punishing traps and annoying gating (often low-level) roamers, and lacked differentiation between areas (or groupable named with loot worth mentioning!). SG just kind of felt unitemized and unfinished in the way Chardok was before its Velious revamp.
4) Plane of Fear
I loved the pre-expansion raid game in EQ. Although content was limited and somewhat flawed, I don't think the developers managed to improve on the experience, at least for me, until PoP.
It's hard to beat ToV, which I see everyone else picking, for atmosphere, but a raid zone - a whole raid expansion, really - in which *everything* is immune to mez and charm and in short *all* the stuff you've learned to do with your hard-won skills learned in dungeons groups leveling up, flat out sucks. Although Velious was in many ways a big step forward for the raiding game in EQ, I think the original planes of Fear and Hate were more fun at level 50 than just about any Velious raid at level 60.
Fear and Hate (also the usually-two-group, at least on the break-in, Efreeti Lord Djarn camp at level 50) were like multiple-group dungeon crawls with the difficulty, the risk, and the reward all ratcheted way the hell up. You could do all the things you could do to a frog in Lower Guk to a PoF mob, but these mobs were deadly hard - remember that a good piece of gear at this point in EQ is a piece of gear with +5 int on it; people are wearing armor made out of cardboard and trying to bludgeon these mobs to death with feather dusters - and aggro'd at range and in numbers. And, of course, they come with a range of nasty little abilities like shadowstep and fear procs.
The armor was also easily identifiable and a very cool status symbol. Rangers in green plate!
I picked Fear over Hate, though I liked both, for the atmosphere (hate is a little bit all-the-same) and because it was a bit less broken - it sucked to wipe to a geometry bug in hate.
The one thing I could have done without in fear (besides those @#?*@ dispelling tentacle terrors...) was the combination of deathtouch with telephone aggro. Though even that had its entertainment value! Remember the serial-trainer named Boneman I mentioned above?
5) Plane of Air (PoP, not sky)
I know this zone isn't popular with everyone, but I love it. Really challenging crawls through an interesting zone on a tight timer - Air Rings are fun for me. I didn't like them at all the first time I did PoP (on PC), weirdly, but they've grown on me, I guess, and I probably have the most fun with these out of any of the EP farming we do, with the possible exception of the Doomfire Ring.