Welcome to the site, and to forum posting! 
A lot of it comes down to the condition of the particular machine and how well it was cared for.
Out of the list you mentioned, I think I would stay away from the box store brands such as MTD, Murray, Troy-Bilt and Poulan, especially if it's going to be bought used. You will get a lot more quality for your money out of the name brands such as Deere especially and Craftsman, depending on the models. Like I said though, a lot of it really depends on the specific machine's condition and how it was cared for. You could find a really nice well taken care of MTD that looks almost new that is in better shape than a used JD.
I would pay a little more and stick with a name brand. Find one that is in nice shape that has been well taken care of. They are also easier to get parts for and the general rule of thumb is that you get a lot better quality out of them because they were built to last.
When you go to look at them ask the owner questions like:
1. Are you the original owner?
2. Has the machine been stored indoors or outdoors?
3. What maintenance has been done to the machine? How regularly was the oil changed?
4. Check the condition of the belts and blades. Look for rust in and around the mowing deck.
5. Test drive the machine and check it all out; PTO engagement, confirm is steers & drives correctly, brakes and parking brake works, starts and runs as it should etc etc.
A lot of it comes down to the condition of the particular machine and how well it was cared for.
Out of the list you mentioned, I think I would stay away from the box store brands such as MTD, Murray, Troy-Bilt and Poulan, especially if it's going to be bought used. You will get a lot more quality for your money out of the name brands such as Deere especially and Craftsman, depending on the models. Like I said though, a lot of it really depends on the specific machine's condition and how it was cared for. You could find a really nice well taken care of MTD that looks almost new that is in better shape than a used JD.
I would pay a little more and stick with a name brand. Find one that is in nice shape that has been well taken care of. They are also easier to get parts for and the general rule of thumb is that you get a lot better quality out of them because they were built to last.
When you go to look at them ask the owner questions like:
1. Are you the original owner?
2. Has the machine been stored indoors or outdoors?
3. What maintenance has been done to the machine? How regularly was the oil changed?
4. Check the condition of the belts and blades. Look for rust in and around the mowing deck.
5. Test drive the machine and check it all out; PTO engagement, confirm is steers & drives correctly, brakes and parking brake works, starts and runs as it should etc etc.