Ok, here goes nothing.
I have been avoiding threads like these because it's always the same crap. Whatever motor someone is running is the best motor on the planet, unless they are having problems with it, then it's junk. Often neither of these are true. So the following is just my own personal opinion based on 25 years of RC racing, the last 10 of which have been mainly nitro. If you disagree, fine. Your experiences will be different from mine so go with what works for you.
Novarossi: Whether an actual NR or a rebranded motor like RB, these are the highest quality, most reliable motors you can get. Don't believe me? Ask any 1/8th onroad racer where Nova motors dominate not only for their performance but for their trouble free, reliable, easy to tune nature. Sure they have made the occasional dud, everyone does but your chances of getting a good one are much higher. Strongest rods in the business, PERIOD.
Star/Sirio: I have a hard time recommending these because I have had issues with them. That being said, the quality of manufacturing and performance were second to none. I just feel there are one or two areas they need to work on to truly match Nova. Being a much younger company than Nova, it would not surprise me if they get there.
Picco: I have weird feelings about Picco motors. I love the way they run. I love their generally lower price. The problem is that quality seems to vary with every new motor they come out with. When Picco gets a design right, they are as good as they come. Then they make a bunch of changes for the next motor and you get lousy carbs or thrown rods or broken sleeves. Despite this, I'm always willing to give a Picco motor a try. When they are right, they can really bring a smile to my face.
OS: I feel much the same way about OS motors as I do Piccos. The things just run the way I like a motor to run. They are the only manufacturer that can match Nova (and maybe even beat them) in the carb department. The only knock is longevity. Rods, piston/sleeve, bearings and sometimes even cases just don't hold up as well as some others. They aren't terrible in those areas and in fact are much better than some but not as good as they could be.
All others: Some are absolute garbage (one brand with three initials as a name comes to mind) some are actually pretty darn good motors. I leave it to you to decide which is which since I'm not running them until someone beats me with them on a regular basis. Then I might give one a shot.
Note about all others, I'm not talking about motors like Werks or Orion or whatever here since those are all made by one of the manufacturers listed above. I'm talking about the actual manufacturer.