1) Amiga - Because it was an awesome machine to code for and had loads of games.
2) Super Nintendo - loads of great rpg's came out on that system
3) Sega Megadrive - whilst i preferred the snes. there were still some good games on it.

1. Dreamcast
2. Arcade
3. Neogeo

1. Super Nintendo
2. Dreamcast
3. Nintendo 64
With an honorable mention for the Sega Genesis, because that is the one with the best odds of actually coming here (since it is already on Steam :p).
SNES because I like a lot of the games on it.
Dreamcast because it most fits in with the style of the site. Games that were overlooked at the time, but that were actually rather good once you got past the rough edges.
And N64, because Perfect Dark and Mario Tennis alone will probably bring a crapton of people to the site.

Greetings to the Team !
1 - Super Nintendo, for her legendary games ;
2 - Dreamcast, for her "computer spirit", and for her weird games like Space Channel ;-)
3 - Megadrive, because I won't play much with her as I was young, but I love this Sonic spirit !

1. Commodore 64
2. Amiga
3. SEGA Dreamcast

1. Amiga
2. Commodore 64
3. Dreamcast
I love Genesis and Super NES, but unless you can get the rights to some games that haven't already be released several times over, these are my choices. I have Virtual console, XBLA, various collections including Sonic's Ultimate Genesis Collectin (which I'd recommend to anyone with a PS3 or 360.)
I'd also like to add Apple //e, but that's just nostalgia from school.
Though, if you could get something like Sonic CD working (Either through emulation, or fixing the PC port.) I'd buy it.

1. Amiga
2. Commodore 64
(3. PS1 if I were capable of choosing that)
Nintendo is still milking their franchises like mad, easy to get similar games/remakes of many Snes classics on their handheld.. or on Virtual Console.
Arcade games are mostly shooters + fighting games.
Give me the real deal - old games from an area where PC was a "personal computer" and not a soulless Intel + MS clone.

I would like to see SNES, Geneesis, and the Commodore.

1.Sega Dreamcast
2.NeoGeo
3.Arcade
I have to go with with what Thimoten listed because I think the Sega Dreamcast was too far ahead of it's time and had some brillant games but are hard to come by now. Not sure how you would emulate it though if you are allowed to that is.
NeoGeo Because that system has the awesome Metal Slug series and King of Fighters Series, plus so many wonderful 2d games.
Arcade is the original classic gaming system in my opinion. Maybe you could with permission release Arcade Gaming packs by Developer. Eg: Konami Arcade Pack. With perhaps 20 or 30 games per pack as the size of some early Arcade games was very small. That I think would be awesome.

1) Arcade
2) Dreamcast
3) Commodore 64
Most of if not all the others are on various other services eg. Virtual Console so it'd be nice to have variety.

1 - Sega Genesis
2 - Super Nintendo
3 - Nintendo 64
I would also add Apple ][ to the mix if possible :)

1. Super Nintendo
...it has a higher density of really good games than any other console I know, and has NEVER been available for purchase in my part of the world :(
2. Gameboy Advance
...not on the list, I know, but I'm in love with Megaman Zero so I don't care
3. Neo Geo
...Metal Slug FTW

1. Nintendo 64- my first love
2. Dreamcast- missed it when it first came out, really want to play some Sega classics
3. Snes- brilliant games catalogue, a golden light for retro gamers

1) Amiga
2) Commodore 64
3) SNES
Because these are the systems I used in my youth. The Amiga has lots of games that look better on it than on the PC (especially pre 1990 ones). Commodore 64 is probably the most popular 8-bit home computer with a big software games.

1) Mega-Drive / Genesis
2) Super Nintendo
The two big console systems around when I was a kid. A kid who didn't have any income and missed a lot. While I did have the first one and manage to grab a lot second hand later on, it has started to show again on other DD services and sounds like a better possibility. And the competing SNES was something I never owned.
3) Amiga
More out of curiosity than anything else. Even seeing different ports of DOS/Amiga would be neat, as there are differences between the same game in some cases.