1. Arcade
2. Sega Dreamcast
Because you can find and buy Super Nintendo, Sega Genesis, Neo Geo and Nintendo 64 games in the Virtual Console of the Nintendo Wii. I would gladly pay 5.99 dollars for the Arcade games, and 9.99 dollars for the Dreamcast games.

1.) SNES
2.) N64
3.) Arcade

1) Amiga - Its almost impossible to ifnd amiga games anywhere else, has a huge back catalogue and there are so many classic games I'd be happy to buy again!
2) Arcade - Again, huge back catalogue and a slick wrapper for the games would get me to pay, although perhaps not as much as for amiga games!
3) Neo-Geo, probably the only platform I don't have experience of, so would be interested to try out a game or 2.
As for the others, they are either not that old (I still have my N64 and Dreamcast working fine) or widely available elsewhere (Wii Virtual console, XBLA, even Steam now for NES/SNES/Megadrive) so there would be a lot more competition. If you could put them out at a very low cost, then any of the others would be good (I'd even consider rebuying N64 games I already own at ~£1/$2 each, but it seems unlikely they would be anything like that cheap).

SNES
Dreamcast
Arcade
For me, the single biggest reason for SNES is the squaresoft titles. Dreamcast has a number of titles in several genres that are pure classics: jet grind radio, space channel 5, skies of arcadia. And of course arcades have had strong titles since before I was born

1) Dreamcast
2) SNES
3) N64

1. Super Nintendo
2. Sega Genesis/Mega Drive (Might have been higher, but there are so many Genesis collections.)
3. Arcade (Especially Capcom titles)

1. Super Nintendo
2. Sega Dreamcast
3. Neogeo

1. Amiga
2. Commodore 64

1. Amiga
2. Neogeo
3. Sega Dreamcast

in no particular order:
Sega Dreamcast
Super Nintendo
Neogeo

I almost feel it is a waste to say Nintendo products due to the nature of the company, not really even the slightest possibility of happening.
1. Arcade
2. Neogeo
3. Megadrive

1. Amiga
2. Neo Geo
3. Arcade
Mostly because these are hard to come by in any system. I'd pick Super Nintendo or Genesis but these are widely covered even in Playstations...

In no particular order.
Sega Dreamcast
Arcade
Neogeo

1. Amiga
2. Commodore 64
3. Amiga

This is my first post here. So, first of all, nice to meet you all.
My votes are for:
1. C64 (TAI-PAN is still in my dream)
2. Amiga (Defender of The Crown, Utopia, Power Monger... so many titles)
3. C64 !!!
I will pay money (not too much ;) ) to download safe and easy-to-use emulators, as well as surely-running games, rather than going crazy with tons of settings, suspect downloads and so on... and, as someone was saying, emulation is the real basic of retrogaming.