Since I have owned, or still own several of the mowers you guys mentioned, I should add my opinion.
A) And the winner of the best all time 21" mower ENGINE is:
Snapper Mower with the 2 stroke Wisconsin Robin engine. The reason being is for the commercial unbreakable feel. The ability to run many years in a commercial application. This mower engine combination, when used to bag fall leaves, acts like a vacuum cleaner with impressive suction. No leaf, stick or twig remains on the ground and is safe. It would probably pick up a quarter from the ground (and give you change back). Cons> Very heavy. Also, the pull chord takes so much effort to start, it will wear your arm out, if it doesn't start by the 3rd pull. For this reason alone, I sold mine (even though it had less than 100 hours on it).
You occasionally still find this mower/engine combination for sale on craigslist.
B) The winner of the best 21" mower of all time is:
Toro mower with the 2 stroke Suzuki engine. This is the one which made landscapers mourn, when they were no longer made. Able to last many years in a commercial application. The cons> If you need a new coil, be prepared to pay around $100 for one.
I have one, that was restored by a MidWest Toro dealer in like new condition. It is more of a mower for show, not a "daily driver." This particular dealer makes a nice profit restoring these two stroke mowers each winter.
Second runner up goes to Lawn Boy with the E series, 2 stroke engine. With it being 6 1/2HP, it is the most powerful 2 stroke 21" mower engine made (maybe 4 stroke too)! It is real sweet, has a high power to weight ratio (and weighs no more than 70 lbs). I own one with the commercial Orange 1 gallon gas tank, and orange, steel wheels. It also has a 3 speed transmission. Lightly used ones on eBay have been for sale for over $1000.