Believe it or not, I always ran the OEM steel cage Koyo bearings in my own engines. (The ones that are standard/stock in a new Yamaha). They are actually a very nice bearing.
When I was in the engine building business, most every customer insisted on a "better" bearing (more accurately: what was *perceived* as a better bearing). Consequently, I had tons of new Koyo stock bearings in my shop that I had removed from brand new engines/cases that I was blueprinting.
So... that's what I have always used.
The perception in the engine building business is that a "looser" bearing is faster (in other words: a C4 bearing instead of a C3). There are a couple of issues that pertain to that: for starters, the specification for internal clearance in ball bearings has a significant "overlap". So much so that a C3 bearing on the "loose" side of its tolerance actually has more internal clearance than a C4 that is on the "tighter" side of its tolerance. So right off the bat: a bearing needs to be measured for internal clearance in order to "really know" what you have.
The next issue is: if an engine case is accurately machined (bearing bores perfectly in-line, and back faces perfectly parallel), then main bearings with more internal clearance are not required, and may actually be a detriment to how precisely the engine is rotating.
That said, the nicest bearings I ever "felt" were any of the FAG bearings that are actually made in Germany. Difficult to find these days.
PM