Most of mine already came:

1. Ultima series
2. Master of Magic
3. Syndicate

The only thing missing in my life now:

4. SSI D&D RPGs (Dark Sun 1 and 2, Ravenloft 1 and 2, Dragonlance 1, 2 and 3, and so on).

Many of my "insta-buy" games are already available on GOG, but...

The Chaos Engine. I loved this game on my Amiga back in the day.

Also, I think most of us would die of pleasure if the LucasArts adventures showed up. There are some available on Steam (Monkey Island 1&2, Indiana Jones, Loom and The Dig IIRC), but I only very rarely buy anything there because of the DRM / Win7 compatibility issues with many of their older games (which is what I play mostly).

I'd love to see more Sierra titles as well, especially the Quest for Glory and Leisure Suit Larry series. Shivers 1&2 would be awesome too.

Age of Empires would be nice, even though I already own that physically. And Empire Earth is pretty similar (I just got that in the last promo, thanks GOG!).

More classic Epic titles such as Epic Pinball & Jazz Jackrabbit. I played the hell out of those when I was a kid and I'd love to give them a spin again.

Uh... That's all I can think of right now...

Legacy of Kain series, the whole lot of it!

Hey, you guys after the Homeworld games?

Have you tried Haegemonia?

SNAZZY!

Instabuys :
Ughh! Far too many.
Theme Hospital.
Age of Empires 1.
VtM:Bloodlines.
Diablo 1.
MIA:Missing In Action.
Starfleet Command 1. (I never got around to 2 or 3, so maybe even those).
Star Trek : 25th/Judgement Rites.
Mechwarrior ....2 (I think it was).

That should do yous for now...

The games I instantly bought from GoG was Fallout, Fallout 2 and Fallout tactics. Then I bought Ground control 2 after I played the demo.

I love Jedi Knight Dark Forces II. Been years since I played it. It would be cool if GOG offered a set sometime in the future.

As far as other insta-buys non come to mind at the moment. SSI Goldbox collection would come close though.