I would vouch for the Geo Metro, but a "sleeper" nobody has mentioned is a 1988 Oldsmobile Cutlass Ciera with a 2.5L "Tech4" motor.
It's more of a midsize car, 30mpg+ consistantly, roomier too, important if you are on the big side.
Now talking about the Prius...
It TOTALLY kicks the crap out of a Prius!
The metro is bigger, has more legroom, more cargo space.
The metro is WAY more plush inside, and with its Nav system you never get lost.
The metro has way more power then the Prius and accelerates much faster!
The metro also will protect you way better in a crash then the tin-can Prius, cuz the Metro has dual stage airbags and side curtain airbags.
No contest!
Rusted out Metros go on craigslist for $800 and used car dealers buy em same day and resell em for $2500 to fat people with bad credit.