Off-The Shelf
I'm partial to old 70's/80's MTD/Wards stuff for push/self-propelled, particularly the 300 series stuff (362, 364, 343...etc), so as long as it has an 110000 Series Briggs on it. The thing about them is though they are "cheap" they have an advantage of me being able to cobble new parts for them from stock at the hardware store for cheap. I'm building a new rear axle for the one I have now.
I also like Frankenmowers a lot. Years ago I had a Foam Green "Mur-T-Boy" - an 88 Murray with a Briggs Max, MTD Wheels in front, and Lawn-Boy wheels in back. I also had the "Candystore 20" - a 20" Murray deck with a vertical pull start 3.5 HP Briggs engine, ball bearing wheels, and an MTD "shopping cart" handle - painted a freakish white/orange/yellow paintjob, it look like a fisher price toy turned into a real mower. Man, I wonder where the photos went of those crazy beasts are.