My choice would be the WR250X.
It is the only fuel injected bike. Has nearly the same power as the DRZ but a wider range 6-speed transmission which is helpful on the highway. It is very light and seems to have the best suspension of the list. If it matters this bike should also get better gas mileage than the bikes with larger engines. This is the bike currently at the top of my list for when I decide to buy a new bike.
However all this comes with the highest MSRP (about the same as the DRZ) and it is a recent model introduction (’08) so the used market is limited.
You are likely to find some good used deals on the GS500F and the feel of the bike will be very different. Go to a few dealers and sit on the bikes. Get a friend or sales guy to hold them up so you can get both feet on the pegs. This might sway you one way or the other.
I think any of those listed are a good choice.
Craig