for sheer availabilty and knowledge, Yanmar is the best , fitted to moat european production boats. Beta makes a good engine , but really its a small niche UK manufacturer. Perkins is fine
fir reconditioned stuff, but really hasnt a new design diesel. Cummins is really a US make, hard to get parts elsewhere. (good engines though, bit like Scania in Europe) Volvo is OK though there has been...