I would also opt for the GM piston. However, keep in mind that parts for them are all NOS or from parts machines, and can be hard to come by. Your best bet would be to acquire a "daily driver", something that's easy to fix, with parts readily available. I may sound something like a rebel here, but you really can't fault the shredmores for their reliability. They actually do clean very well, probably better than the belt-drives. If used right, they aren't as hard on cloths as one would think. Get one that was made prior to 2006, and preferably one that was made back in the '80s and '90s. Then, get the GM Frigidaire for use on stuff you know won't tear it up. That's what the shredmore is for!
On GE Filter-Flo's: I like them as well as I do the shredmores, but they are really not as effective on items that tend to billow. Parts availability is going to be an issue one day, as GE stopped making them about 15 years ago. Other than that, they are a great machine and probably among the most sought after by collectors. If you find one, keep it.
Belt-Drive Whirlpools and Kenmores: They they share some of the GE's traits on billowing. Although they stopped making the belt-drives before GE did away with the Filter-Flo, parts tend to be easier to find. These would probably be your best choice for an older machine to use regularly, because they are far more plentiful than the GEs.
Maytag orbitals: These are among the most common of the classics. If you check Craigslist, then you are bound to run into one on any given day. There are many folks on this site who like them, and would be glad to help you find the parts you need when doing a restoration. They are among the most reliable workhorses, but rollover is not as good as with the GEs or Whirlpool surgilators.
NorfolkSouthern