It shouldn't matter what a console sold it should depend on the games. The NES outsold the C64, ZX Spectrum and Amstrad CPC not to mention the Sega Master System and Atari 7600. Does that mean only the NES should be considered? No.

1. Arcade
2. Super Nintendo
3. Nintendo 64

1. Arcade
2. Neo Geo
3. Dreamcast

Neo Geo
Arcade
Megadrive

Dreamcast. Fix it so I can play Seaman on my PC plz.

1) Dreamcast - 'cos it's the best damn console. Ever.
2) Commodore 64
3) Arcade (depending on the games that were a possibility)
I'd also pick N64 and Genesis/MegaDrive, but these are all options available elsewhere (Wii VirtualConsole and the likes). RareWare games would be awesome, and if there was any chance they would be a possibility, Nintendo 64 would be my second choice, but with all the licensing hell on their older stuff, it would be pretty much impossible (well, very improbable)
I know the C64 is on Virtual Console elsewhere in the world, but down this way there's no way to legally acquire the old stuff. Although, I still have my C64 and original games, so my purchases would probably be limited to the stuff I don't have, unless there's a good enough reason to buy it again (extra content and the like)
Arcade would score higher if some of the slightly older 3D stuff could be licensed. Anything NAOMI hardware based would be kickass, and wouldn't be too hard to emulate on modern hardware - especially since the last dreamcast emulators to be updated were starting to get fairly decent compatibility. Any of the Sega arcade games would also rock, although the mid-era system hardware is slightly more difficult to emulate, just like any of Sony's arcade hardware. The newer Sega systems would be a sinch if you could license the games, the lindburgh upwards are standard PC's with a couple of hardware checks for DRM/Security.
Arcade wouldn't count for me if the games featured were the standard 2D faire you can get on other platforms (except for a few exceptions). I know they are easy to emulate for a wide range of users regardless of underlying hardware, but if it's available elsewhere I probably already have it, and I'm sure the same goes for the bulk of the systems.

1. Dreamcast
2. Nintendo 64
3. Amiga

All 68k based systems (Megadrive, Amiga, Atari ST etc.)
Super Nintendo
Sega Dreamcast

Turok, Banjo Kazooie, Banjo Tooie, Super Mario 64, Perfect Dark.
Nintendo 64 is all you need.

1. Arcade
2. Dreamcast (Seriously, I have no desire to find a used import version of Radiant Silvergun for $150).
3. Though it is not a system and has been requested before - KoTOR 1 and 2. Both legal copies I own keep crashing on my XP system. Also not a system, but what about the stuff Working Designs used to import? Atlus does a decent job now, but lots of old Japanese games never saw release outside the Land of the Rising Sun.
lastly, Nox and Planescape Torment also would rock. Thanks for listening.

1. Amiga (best computer EVER ;o) )
2. Nintendo 64
3. Sega Dreamcast (I never had one but so many people say it's great that I'm curious)
--
Unlisted platforms that might be interesting to add:
- Intellivision (my first love ^^ )
- Atari 2600
- Amstrad CPC (lots of great games, although most might be present on other systems)
- Sega Master System
- NES
- Playstation(s) (PS1 is probably old enough, not sure about PS2)

1. Dreamcast
2. Amiga
3. Neogeo
I wouldn't pay much money for SNES, N64, or Sega Master games. There are "alternative" solutions for those systems that work extremely well. However, if they were fairly cheap (NOT $6 per game), I would buy them just to support GOG, especially if the titles came with box art, instruction manuals, and soundtracks (in mp3 form).
I don't know how you guys could pull off dreamcast, but I would dish out some serious dollars for that, ESPECIALLY JAPANESE IMPORTS.

1. Dreamcast
2. Arcade
3. N64

1. Arcade: Porting over hundreds or thousands of old and new arcade games would not only be a dream, but it could really have huge potential to attract a lot more customers who are looking for more than just old pc games.
2. Amiga: Also a very large selection of great games. Amiga titles would be a great supplement to old pc games, and there are a lot of great classics people would love to play again.
3. Neo Geo: More-so of a personal preference of mine. I would love to see the Metal Slug and King of Fighters series' come to gog, not to mention the many other games created for the platform.
*Just on a side note: I'd also like to see Dreamcast games make it's way through as well, but my first three choices are my favourite.