Out of all the guns available in the department stores, I believe the Ruger AirHawk to be the safest bet. And only because it is a copy of the Diana RWS34. I had one for a couple years and was a good gun after I tinkered on it a little. Get one and tear it down and replace the piston seal and lube it properly, and it will last you quite a while. Parts are readily available (since it's a clone of a popular German rifle) and cheap. You could even slip a tune kit in it from Vortek or ARH ( Maccari). All that and still come in around or under the $200 mark.
The Ruger Air Magnum is a similar rifle to the Airhawk, but has considerably more kick and will most likely not be as accurate. It is a clone of the RWS350. Tuning parts are available just like with the AirHawk though, so you can tinker it to your liking.
I haven't seen anything great about any of the Stoegers or Gamos to be honest. With the Ruger guns, at least you are getting a copy of a well established, well designed gun and it's made mostly of steel. For the money, the AirHawk with a little work would be my choice.