Honda if you want it to last for 15 - 25 years.
Briggs and Stratton if you want it to last for 5 - 10.
The most important thing is oil changes. If you do not change the oil, the mower will not last. You have to understand that there is no oil filter in these small engines, and if you do not change the oil every 25 hours, you wear things out very quickly. Changing and/or cleaning the air filter is important too.
Of course, I'm just talking about engines here. I don't know who makes the best mower decks.
<--- resident ATOT small engine nut.