Wow.
Another four years of Dub. I think Eric summed up my thoughts well enough.
It's going to be hard, but I'm going to try and talk about something more fun and happy-orientated now.. NWN..
*deep slow sigh*
Regarding "expansions".. I'm assuming you mean modules and not expansions for the sake of my response.. (expansions = Urentide and Underdark?)
It's very very close between the Twilight/Midnight series and Honor Amongst Thieves for what I'd consider the top two modules in the entire database. They are both incredibly innovative and amazingly well done. I guess it would come down to "mood". If you're looking for something less linear and more open to free-choice and resulting "endings", then "HAT" is the one for you. If you're looking for a more direct storyline with some incredible interaction between your Paladin character (no other options here) and your DROW henchman/squire that you're trying to counsel and bring up from her chaotic/evil ways.. then that's your animal.
My travels took me through the Penultima series immediately after the original campaign which was a lot of fun for the completely different feel of a "funny" module with a great storyline. The custom items in the module are frigging hilarious.
So, I'd say one of those three for your starters depending on your mood. I sincerely don't think you can go wrong with any of the three.
Get them while you can before Ashcroft bans them as Satan's works.
*sigh*
Late,
~O.