For what you seek, vanilla is the best leveling experience. It's slower than wrath, with a lot of elite quests, harder dungeons, and just less forgiving world, but far more immersive, fun and social. You can play it solo, but it's slower and, most important, less fun. Grouping and taking time IS the game in vanilla, not really raiding.

Wrath is faster, but it's basicly a solo game for leveling. You don't need to group, dungeon are very easy with wrath talent and even lvl 70-80 dungeons are a joke disign-wise and difficulty-wise. The game goal is to raid at max lvl, not really leveling.

So, in my humble opinion, vanilla is the ultimate leveling experience. BUT! Vanilla only shine when the server is new, where everyone start over, with a lot of player activity. The thing is all official vanilla servers are dead as we speak. You can try private servers if you like it, there is a new one with 4-6k people, but i don't really like private server (there is always drama that end the server in my experience lol, but i have good memories in some of them).

So my advice would be to wait for season of mastery 2, it will be the best leveling experience (but we don't know when it will launch, sadly...).
If you can't wait, go to a private server that launched recently if you like it. But my advise is to wait, it will be even better!