You’ve asked a question that is sure to elicit many passionate answers. It’s kinda like, “what kind of car should I buy”. OS engines are still pretty good and some folk swear by them. I have also had good performance with MDS, Magnum, Norvel, PAW, Fox, and K&B (and even an old “Bluebird” that I got on e-bay). The prices are competitive and a difference of 10-20 dollars is not much when you will spend several hundred for the total project. The main decision is whether to go for a bushed engine or a ball bearing. The bushed will be cheaper, won’t require as much after run care, and will give adequate performance if you choose an appropriate size for your airplane. The bearing type engine is a little more expensive and will develop more power in the same displacement but some after run care is required to maintain bearings. The fact is that all the major manufacturers produce quality engines that will give good service.