I can't say which is the best one, all in all, but I think that BW Razorclaw should be able to give anything a run for its money based on just how neat such a small, cheap figure is. He was either my second BW figure or joint first with Airazor (which is also outstanding but slightly less to my preferences). This is the sort of stuff you hook a kid on a toyline with.http://www.tfu.info/1997/Predacon/Razor ... orclaw.htm
Neither of his decos are great because the orange and teal respectively clash with the rest but the sculpted detail is very good. The crab mode isn't the most exciting because, well, it's just a crab with oddly large hind legs. The transformation is simple but you still end up with an impressive-looking robot mode, especially when you take into account that his claws house two hidden weapons. The large one has serrated spikes that pop out when you pull a little lever and the small one has a silver-painted blaster. Of course, they're also fully usable in beast mode so I suppose that the crab can be a lot more exciting than first thought.
Basically, as a toy, this guy is amazing. Articulation, a truly excellent gimmick, no accessories to go missing and he's cheap. Too bad his colours are so-so. The video version is a little nicer but way more expensive. As a last note, the way his head works means that the beast mode has crab mouth gnashing action. That is attention to detail.
As for beast transformers in general, there is one thing that I think makes up for the awkward issues they can have: uniqueness. Basically, there are hundreds of car transformers. Even less common things like motorcycles, helicopters and tanks have dozens each. Beast modes are by contrast a lot more tied to a character. You have plenty of different characters that are some variety of plane but if you want a rat, an elephant or something odder like a squid or rabbit there is pretty much one character each for those. Finding a new kind of animal is admittedly a lot easier than new kinds of vehicles and there have also been far more vehicle transformers so it might not be an entirely fair point to make. It still adds a lot of novelty to beast modes, though, even ones that aren't done better than a high-quality standard Cliffjumper or whatever.