Back in the day (1982) I started up a full range cleaning business - janitorial, floor care, carpet cleaning, window washing, etc. During the last 32 years I've had the opportunity to use and own MANY vacuums. Here's a quick overview of the list that I can remember (I'm probably forgetting a couple).
When I first started my cleaning business I bought a rebuilt Kirby. It was a solid workhorse. But I was never convinced that the Kirby was the ultimate dirt collector - it only seemed average as far as performance goes. And it was a fairly dirty vacuum, as all cloth bag vacuums are.
From there I moved on to a Sanitaire which I felt performed much better than the Kirby. What I did not like about the Sanitaire is that although they suck dirt well, it's also a DIRTY vacuum (as was the Kirby). It spewed a lot of dust through the bag and back into the environment. And it was also easy to break stuff on them... fan, handle, foot lever, belts, cord-wrap, etc.
I also had a Panasonic commercial back in the early years. I liked that one. The Panasonic was cleaner than the Sanitaire and Kirby. It also was more durable than the Sanitaire. Performance was adequate - maybe slightly less suction than the Sanitaire, but more than the Kirby.
And at one point much later, I owned a commercial Royal. I liked the Royal a lot at first. Performance was on par with the Sanitaire. It reminded me of a sports car version of the clunky Kirby. And though it worked well at first it got ugly and corroded from the moisture in the van very quickly. And like the Kirby and Sanitaire it was also a DIRTY vacuum.
For a while I was enjoying the Hoover Conquest vacuums. Now that's a seriously strong commercial vacuum. Built like a tank. Great suction and burly enough to pound nails with. However it was also a moderately dirty vacuum and it's heavy to work with. It really is like vacuuming with a truck.
I've also worked with Dyson. They clean well. But I can't stand the cheap plastic toy-like build quality that they have. I've also owned a couple of Oreck's. Oreck's clean reasonably well, but they're also toys - and they're dirty.
Through the years, I've opted for dual motor commercial uprights like the Sebo/Windsor, Lindhaus RX 380, and ProTeam ProForce 1500xp Hepa vacuums. These vacuums are all HEPA vacuums and therefore are uber-clean (they don't spew dust back into the environment). And the dual motor design performs cleaning very well. These days I'm liking the bang for the buck that the ProTeam ProForce 1500xp Hepa delivers.