best 4 stroke engine?

Would help to know what kind of build you are considering, in frame or rack mount, and what the speed and load ranges are. There are a bunch of great 4 strokes, some are easier that others to build around.

All I can say is, I have a 79 cc / 2.5 hp Harbor Freight Greyhound engine (4 stroke) and it is fit as a fiddle. Pulls strong and isn't as big as other industrial type (Briggs & Stratton) 4 strokes.

Depends on what you want in terms of size, IE displacement/HP, dimensions and weight.
Any discussion of best 4 strokes would include the Robin and Honda 35cc engines. Honda also makes a 50cc class engine though personally I'm not a fan of wide, heavy engines and prefer the 35cc class for MBs.

These are the internal combustion engine which has do 4 works to complete 1 cycle basically these engines are being used in motorbikes which gives better performance comparative to 2 stroke engines.

And lets not forget sound a lot better than 2 strokes ! Truth of the matter is is larger 2 strokes (125 ccs and up) put out more HP per cc than 4 strokes because there are more power strokes of the piston per RPM. But for some unknown (except for a fisicist maybe knows) reason " little " 4 strokes put out better usable power per torque than little 2 strokes (unless 2 stroke is made high performance). Little 2 strokes can put out unreal power per cc if tricked out. But the sound of them in this state of tune is pretty much unbearable IMO. 4 strokes are heavier per HP but put out more usable torque an easier to live with. And lets not forget, usually a lot more dependable and sound more pleasant than a 2 stroke.