I got a year-old John Deere GX 255 Garden Tractor last month, and it is the GREATEST mower I've ever had! The frame is very stiff, and everything about it seems very tight and very well-designed.
Spend a little more and get a year or two old John Deere GARDEN TRACTOR and you will have something to last for years. Mine is way tougher and better than the Toro/WheelHorse it replaced, and it is way better than anything at Sears, Lowes, or Home Depot. (Don't be fooled by the John Deere's at Lowes or Home Depot--they are made in the non-union plant in Tennesee and their quality is nothing compared to the regular John Deeres.) (I don't care about unions, but this is a good example of the differences in quality/price between union/non-union.)
Check out the link at the bottom for comparisons between John Deere models to see which one to get. Note there are two more links at the bottom of the chart on this page to see the Garden Tractors.
I guess if I had to buy the cheap ones, I would look at a Sears DGT 5000 or 6000, or a high-end Husky or Poulan Pro, but these are not nearly as well-built as the good John Deeres.