I think it really depends on what you're doing and how you like to play. There's a lot of addons other people consider essential which I'd never want to use and others I wouldn't be without which it seems like hardly anyone else has ever heard of.

My approach is to wait until something in the game bothers me enough that I want to find a way to stop/change it, then go looking for an addon to address that. Then I'll usually try a few and pick my favourite because the most popular version isn't necessarily best for me. (For example I prefer addons which blend in with the default UI so I can almost forget what's an addon and what isn't, but that doesn't seem to matter to a lot of people so many popular addons look totally different.)

I think the only one I wouldn't be without which I also regularly see on 'must-have' lists is Advanced Filters, which adds sub-categories to the inventory menu so you can do things like see just your containers and no other consumables.