I think the Secret Sparkles are really versatile. I've used them for more outfits than I ever expected to.
Also I think that small wigs (the buzz-cut and bald ones are best, but even ones that just have very short hair work) are useful for working with the head-modifying items that don't remove the hair, like the Paper Afro and Starhat and such.)
Also personally I use Seafoam Gown and the Babydoll items a lot, but those ones are more of a personal preference and less of a 'this works for everything'. XD
It's probably already been mentioned, but Masquerade is my most used item, hands down.
I love it because it lets me change my mouth to go with what I'm wearing, which makes more of a difference than you'd expect! I have two now so that I can make a more smokey eye look if I want to (like now). Seriously, for all the fuss made over the Agape Scissors on another thread (and here), Masquerade has by far been my best investment, and I bought it with gold before it's price shot up! ^.^
Masquerade. (Buy it. Wear it. Love it.) Note: I've read the entire thread and Masquerade has been mentioned a lot, probably more than any other item.
Humss a very good pair of black shoes, if you have a good pair of shoes, they will go with everything... also an arms mod like the ones in rosmund or s-pop, hums masquerade for female avis(or anyone who wants makeup) and maybe saint ciel agape for the hair mod.
For me also Scarlet mist for the black lipstick, I wear it almost in every outfit,
and a must for every one, bloody and scarry item. something like case of pietro, any scar always make any avatar into an awesome avatar.
Everyone should have at least 5 or so great statement hats. There are some great gold shops hats, contrary to popular belief, and collecting a bunch of cheap little accessories can make a plain avi really pop!