Best in what sense?
The most direct is over the Big Horn Mtns and up to the east entrance; basically US16 the whole way. It's best if your target is the Fishing Bridge campground which is near the middle of the park. East entrance to Fishing Bridge is a gentle downhill, and doesn't have all the hot springs that attract slower traffic.
Continuing on I90 and then south on 191 to West Yellowstone has the most gentle climbs. It also avoids the slow traffic through the park itself.
The NE entrance via US212 is the most scenic, but not for RV wimps.
South via the Tetons is another option, though the most direct still involves crossing the Big Horns.
Keep in mind that Yellowstone Lake is 7000 ft, essentially on a broad mountain top. So there's some degree of climbing which ever way you go.