Pretty much anything with a Briggs & Strat engine. I've had a Craftsman lawnmower for 15 years that I've never changed a sparkplug, added oil, or did anything but change the blade, and it still starts the first pull. My 6 year old lawn tractor also has a B&S motor and starts first turn every year.
^ This.
B&S is hands down the most reliable small engine ever made.
Tecumseh, Kohler, Wisconsin, etc. have more torque per CI, but most I've had will use some oil from day 1 and I feel they are not nearly so reliable.
I have owned 6-7 Lawn Tractors/ Riding Mowers over the years.
Worst ever was a Murray. Poorly made, constantly replacing parts.
Strongest was a Massey-Ferguson 16. Had a 4' "Belly Mower" that was as strong as a Bush-Hog but was good on the lawn too! 16 HP Kohler Engine, always started & ran but had to watch the oil! (This was actually a small "Tractor", w/a 3 point hitch)
Best for mowing & dependability was a Huskee from Tractor Supply, 24 HP B&S Twin Cyl., 42" cut, Hydrostatic Transmission. Smooth running, dependable.(Would still have, but Ex got it in divorce)
Now owner of a John Deere 155, 15 HP Kohler, 38" cut, Hydrostatic Tran.
Been a pile of junk from day 1. Wiring problems, engine so out of balance that it needs to be tightened down monthly from vibrating loose, cheap plastic hood broke, cheap, thin tires, bad deck design. This is supposedly the "Real" John Deere from the dealer, even has the official John Deer plate, but I would rather have the Huskee!