KLX250S or WR250R. Both are cheap, reliable and low maintenance. The WR is a little more expensive, but is allegedly higher performance. Both need suspension work, but not thousands of dollars worth. The KLX needs stiffer fork springs and RaceTech gold valves don't hurt. Go-Race suspension can re spring and valve the WR fork and shock for $800 (maybe more for the springs). The KLX needs some uncorking (airbox, jetting, exhaust) and the WR may need some fiddling with the fuel injection.
Just my $0.02. I don't have personal experience with any of the other bikes. Of course, if I had a Corvette fund, I'd spend it on a KTM instead