1.Super Nintendo
2.Nintendo 64
3.No third choice

Sega Dreamcast
Sega Mega Drive
Nintendo 64

Super Nintendo
Sega Dreamcast
Nintendo 64

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

Yknow what would've made this easier is a proper poll haha.

Dreamcast.
Hands down, and in need of preservation.
Next vote would be Neo Geo.

1. Neo-Geo - Especially Sengoku 3, Shock Troopers 1 & 2, Blazing Star and Samurai Shodown V Special.
2. Arcade - Not the old crap like Pac-Man or Space Invaders, I'm talking about titles from the '90s or '00s that have yet to be released to consoles. Like Alien vs. Predator, Golden Axe: The Revenge of Death Adder, The Simpsons, Battle Circuit, Planet Harriers, Knights of Valour, etc.
3. Sega Dreamcast

1) Sega Dreamcast

Because I never had one and I've heard there was a lot of truly awesome software for it.
2) Sega Genesis
Because if you include the hintbook/walkthrough manuals for some of my favorite games off that system I'd buy them all in a heartbeat. Most of the hintbook game RPGs that're available in retro collections have NO documentation to speak of.
3) Neogeo
Another of those "never had it but heard it had some awesome software" systems like the dreamcast.

1. Amiga
2. Sega Dreamcast
3. Neogeo

1. Nintendo 64
2. Super Nintendo
Having games from these to play on my PC would blow my mind.

1. Commodore 64
2. Sega Genesis
3. Super Nintendo

1) Super Nintendo
Too many classic games to be found on this platform that I would love to revisit some day.
2) Sega Dreamcast
3) Sega CD

1. Amiga
2. Commodore 64
3. Arcade
I think that out of all of these the Amiga, Commodore 64, Neogeo, and Arcade are least accessible to your community, legally that is. The Neogeo and Arcade are easily accessible through emulators like MAME. You could say that about the console titles as well, but the Super Nintendo/Nintendo 64/Sega Genesis games are easily found on the Nintendo Wii's Virtual Console, and there is a huge proven consumer base for those types of small classic gaming purchases.
I think that while classic console gaming is already represented on consoles and handheld gaming devices, you would probably make more money porting these titles versus the older home computer titles that I myself would prefer to see. At the least, I would like to see a continued emphasis on computers with a gradual shift into titles from consoles. Maybe push console titles along with an expansion of GOG porting to other platforms like mobile gaming and *cough* MACS. If you are going to port console titles, why NOT port them to the iPhone as well?

1. Commodore 64
2. Amiga
3. SNES
...for nostalgia reasons.

Commodore 64, there are so many great games that most folks probably don't remember. All the text based RPGs could be bundled up and sold as a package (a lot of them are freeware now, afaik, some aren't): Zork 1-3, Transylvania, etc.
Other great games:
Dungeons and Dragons Gold box games (Pool of Radiance, Curse of the Azure Bonds, etc.)
Bard's Tale 1-3
Wasteland (this may be one of the best games of all time)
Karatika (that may be spelled wrong)
Kung Fu
Defender of the Crown (there is a modern version of this game, now too)
Autoduel (based on the Steve Jackson pocket game: Car Wars)
The Legend of Blacksilver
Those are just off the top of my head, there's probably a ton more.
There's a few goodies for Sega Genisis (Phantasy Star 2-4, original was for Master System), that'd be my second pick.