If you haven't already I'd suggest looking the expansions up on the wiki [wiki.dominionstrategy.com]. It's where I check before I buy one (I've bought a few myself).

Excluding "Intrigue" (great expansion), promo cards (1st editions, etc), my favourite expansions I own are;

Nocturne [wiki.dominionstrategy.com]
(Night cards, Heirlooms, Boons, Hexes, and non-Kingdom cards)

The "Night" cards are an additional stage added after the buy stage. I find the hexes, boons and heirlooms quite a lot of fun too.

Plunder [wiki.dominionstrategy.com]
(Traits, Loot, Events, Treasures, and Duration cards.)

Amongst others adds "loot" cards... n'uff said.

Cornucopia & Guilds [wiki.dominionstrategy.com]

A good sized, varied and fun expansion. Adds cards that give "Coffers" (money put aside for later use) and overpaying. Worth checking out.

Alchemy [wiki.dominionstrategy.com]

Adds "Potion" cards (used for buying cards that needs X-amount of money as well as a potion). Some very powerful, easily abused, maybe even outright broken cards (Scrying Pool?, Possession? in this pack but (imo) can add a lot of fun.

Seaside [wiki.dominionstrategy.com]

Adds duration cards (on your next turn get/do this) and some fun action/attacks.

Menagerie [wiki.dominionstrategy.com]

Adds a lot of "landscape cards" ("Events" and "Ways", cards on the side, not bought but played) a large animal themed expansion. Haven't played much of it yet but has been quite fun.

I own a few more ( Dark Ages [wiki.dominionstrategy.com]) but it's not my favourite. Nothing terrible about it, just "meh".

I can't speak for the rest as I don't own them. I hope this helps and want to stress this is just my own opinion. Best of luck with your choice!