That's probably opening a can of worms. I think you'll find fans for all makes out there, but generally speaking, you can't go wrong with the big three - B&S, Honda or Kohler.
I *think* (do not quote me on this), that the Honda and Kohler run cleaner (better on gas, lower emissions, quieter), than the B&S... I think.. I don't own or have ever owned a lawn tractor