That's a hard thing to ask because, in the end, that's dependent on the taste of the user. Personally, you can't go wrong with owning several colors of an item, several types of accessories and shoes, and so on.
Card Shark bands
(Color) Wool top (layers under most everything)
Blade/Elegant Snowy/Santa Baby leggings
Neutral Starter Urban Girl Pants (they are the perfect length for layering)
Great Old One
Expensive crap worth the price (IMHO):
Case of Pietro
Well, staples for me include Oculus Magica, Oculus Mythica, Superior Form, and Uncanny Form (that last one on another account). I'd love to see them used more. The "dead doll" skin from Superior Form is great and I use it often. The Assorted Contact Lens Set is awesome as well. These are all fairly inexpensive.
I know that, for many Gaians, pets and "I am" poses are favorites--especially kitties. heart
And I'd say skins in general are fun to play with.
But some of the most versatile items are the cinches, reversible bracelets, and reversible hair pins.
gaia_crown I think a lot of the EIs are really useful! Especially the bigger ones, ex: Zodiacal, Compass of Seidh, Case of Pietro, Gogh Reed, Reve Rouille, Masquerade.
Some of the REIs have very useful poses too: Agape (of course), Rosamunds (for the lingerie, if nothing else), Lucie for the make-up.
But really, I think everyone should just be a hoarder like me and never sell anything ever. :B