Too bad i can't say "PC". Because i could wish for some OLDER PC titles [older Sierra's Quests and such].
As for the other platforms, well, i personally wouldn't be interested in buying them, so i don't think i should vote. But I am interested in the emulators GOG would use, and if they could/would help expand those emulators capabilities. GOGs entry to the non-PC area could push development of certain emulators a bit further. Game specific configs for WinUAE would be awesome with a GOG launcher.
And what would be the case of ROMs? Would GOG supply players with Kikstart?
Therefore i vote none.;)

1. Sega Dreamcast
2. Nintendo 64
3. Commodore 64

1. Sega Dreamcast
2. Nintendo 64
3. Neogeo

1. Amiga
2. Commodore 64
3. Whatever

Dreamcast
Like so many already pointed, a great platform that deserved better, and games are hard to come by.
Arcade
Neo Geo
Thinking about the loads of action games with great gameplay and production values that can be found on these platforms.

1. Super Nintendo
As already mentioned, quite some good RPGs came out for this console (Zelda ATTP, Secret of Mana/Evermore, Illusion of Time (Gaia), etc). Emulation of this console on a PC is also proven to be very successful. And of course: Mario (Super MArio World, Super Mario All Stars)!
2. Sega Megadrive (Genesis)
Although I have always been a Nintendo player, I have a weak spot for the Sega Megadrive. It had the Mega CD add-on which was very cool at that time, and the same title on the SNES would look and play very different on the Megadrive console.
3. Nintendo 64
Mainly because of Zelda, Mario 64 and James Bond Goldeneye which I feel were the most decent games on the console that I played.
And I wouldn't mind seeing the 8-bit NES appear either ;)
My only concern about PC emulation is the control scheme: SNES and Megadrive are still playable with the keyboard given a good setup is chosen, but Nintendo 64 becomes problematic with its joypad and all trigger buttons. Users would need to hook up a suitable game controller to their computer.

1. Amiga
2. Atari ST
3. Dreamcast

Mac version of games that already are in the catalogue like Gabriel Knight and Tex Murphy.

1. Amiga
2. Arcade
3. Sega Dreamcast
It's important to note that all the other options are already covered by services such as Nintendo's Virtual Console or the Playstation Network. I'd rather have the others then.

1- Sega Dreamcast (that would be FANTASTIC, by the way)
2- Super Nintendo
3- Sega Genesis

Hate to be a pessimist, but nobody's going to pay for SNES/C64/Genesis games when they can download them for free on a million websites, unless you guys sell some pack of like...20+ roms for $5 or $10.
That pretty much means that you would have to go with the newer video game systems like Dreamcast and N64.
Problem with the newer emulators (N64, Dreamcast) is that they're full of graphics glitches and compatibility problems. To get them up to snuff, Gog would have to modify existing emulators or make their own, which I can't see happening because Gog is still a company and has to make ends meet.

1.- Amiga
2.- SNES
3.- NES

Ok My three are:
Dreamcast - I never had one and there are many games i'd like to play on it.
Amiga - Lots of classics that I only got short glimpses of and friends houses.
SNES - What an amazing console! However I very much doubt Nintendo would ever let those games out of the virtual console.

My, this is an interesting one.
Well, I'd have to say...
1. Dreamcast
Never did play it. Playing it on a PC with no hassle would be great stuff. Heard of legendary games etc etc.
2. Genesis/Megadrive
Pretty much the same reasons.
3. Amiga/C64
Whichever one of those because I'm really not familiar with either.
The reason I won't vote for SNES, N64 and Arcade is beacuse Nintendo has his Virtual Console for Wii - I don't think they want to spread around their old games any other way - and Arcades could get messy with rights and stuff (IMHO)?
Neogeo... Never found it appealing. :3