My most nostalgic zones are definitely Elwynn and Teldrassil, but that's because I played Alliance in actual Vanilla. Those are the zones that really hit me in the feels. But I want to change it up, so I'm going Horde this time. Not much of a nostalgia factor, but a different experience.
So I guess other than that, Hillsbrad! Southshore vs Tarren Mill, good lord, those were great times.
Walking up to the gates of IF for the first time and seeing how massive it is as the soundtrack starts to play. I love it, and its a big part of why im rolling a gnome.
Oh I loved Tanaris, felt like you've really progressed in the game once you get there.
Dun Morogh and Elwynn forest get me every time.
Probably all starting zones? Just hearing this familiar voice and looking over the zone and that finally getting control over you character? Iconic.
Besides those: Tanaris.
Barrens.
Like Grozlam, The Plaguelands, Western and Eastern.
I have a thing about murdering scourge filth, I just wish there were more mobs to kill. I've never seen the scourge invasion, that's one of the events I'm eager to see in the flesh.
Have you played Vermintide [2]? It's the same feeling, you're just getting rid of the baddies around, doing the right work.
Barrens.
I love everything about the barrens until I have to collect the supply boxes from the stupid quilboars...
Darkshore, reminds me those times i play wow for first time as NE male warrior handing around with no mere goal, although it was on TBC but still...
Mulgore, Dark Shore, Thousand Needles, Tanaris, Wetlands, and Duskwood. Every single time. Bonus zone for me is Moonglade, I miss playing my druid in Vanilla!
Dun Morogh 100%
Tanaris
+1, I don't know why but it's such a chill zone.
Elwynn Forest. Always Elwynn Forest.
Also, Winterspring.
Arathi Highlands. I spent an incredibly long time there on my first character, so that zone more than anything encapsulates my memories of leveling in original wow, especially because any alts I leveled I typically didn't take through there.
Tirisfal Glades. There have been many times that I've taken a troll to Tirisfal Glades at level 1 just to level. The atmosphere is so moody and dark, not to mention that it just feels so different from any other zone.
The biggest thing though is coming back at later levels for Scarlet Monastery. It is one of my favorite dungeons and I have great memories of chasing down Alliance trying to flee to the safety of the dungeon portals. It was like a minigame of people popping in and out of the dungeons to skirmish.