N64
Dreamcast
Super Nintendo
Honestly, though, I don't know if I'd want to play them without an authentic controller.
Maybe GoG.com could license out a hardware company to make an N64 and Dreamcast controller for the PC. For Super Nintendo, there's enough out there to replicate it good enough (in fact, I could even use mine for dreamcast -- just not N64).
I found this nifty device:
http://www.amazon.com/Nintendo-64-N64-2in1-Controller-Adapter/dp/8565000168

I would like GOG.com to focus on the PC games. I'm not a big fan of console games, but I would go for any of the Final Fantasy series games & some of the PS1 games.

C64
Amiga

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

Votes summary (only 3 votes per post, restricted to provided list, some margin of error apply):
Sega Dreamcast - 406
Amiga - 327
Super Nintendo - 296
Nintendo 64 - 257
Arcade - 202
Commodore 64 - 193
Sega Genesis - 162
Neogeo - 97

A well timed retrospective on the awesomeness of the mighty amiga (second only to the C64 in awesomeness)
http://arstechnica.com/gaming/news/2010/06/shadow-of-the-16-bit-beast-an-amiga-gaming-retrospective.ars

None.
This would be a waste of resources that will most probably slow the rate at which you release good old PC titles.
I have many old PC favorites that are not yet here, but I can't tell the same thing about old console games. (I know, shame on me.)

1) Dreamcast
2) Arcade
3) Amiga

1. Super Nintendo
2. Sega Genesis
3. Arcade

Wooohoo dreamcast is first, now everyone lets ensure that it will got at least 600 votes.

Megadrive
SNES
Arcade
That should cover most of the truely classic games.

I'm kinda surprised that Dreamcast is on top. Would have expected SNES there. DC has several good titles, no doubt about that but the library is rather small compared to others. I guess the early death of the platform has created bigger nostalgia value for it that usual.
Then again, only reason I didn't vote for DC was my own DC console and library of games that I've bought second hand past few years. :P

I must say i'm quiet surprised not to see ATARI ST in the list. I don't want to exhume an old war between Amiga fan and Atarists but i think that ATARI should have it's place in the top ! Many great games, decent graphics and fabulous music (yes if you like the old style but if you don't what do you do on GOG ? ;) )
Anyway just wanted to point it out, perhaps for a futur addition to Gog's Catalogue ?

1. c64
2. Genesis
3. Arcade
I'd like to see some c64 stuff, if only to satisfy my own nostalga.

While I would be happy with any of them.. the ones I would be most happy with are:
1: Sega Genesis
2: SNES
3: N64
4: Dreamcast
(Whyfor is there no Saturn on that list?)