Best? Well, for me, impressive engineering that takes a realistic vehicle into a nice looking robot wins. So I'm going to say Masterpiece Bluestreak (and friends).

Obviously the MP line has the luxury an adult collector's price tag to work with. So if I'm talking within the kid price tags... heck you could throw a dart into Beast Wars and pick out a good toy... but I'll go with Transmetal Cheetor. Okay, so it's not a realistic alt mode but it's true to the aesthetic of the line & forms two poseable modes with an clever & elegant transformation that integrates the weapon.

Worst? Energon Ironhide. I'm *not* willing to make allowances for the stupid gimmick because there are many examples of gimmicks not getting in the way down the years. It's simply a lazy design... where they didn't even *try* to conceal his head. Engineering isn't clever and realism has been thrown under the bus through a lack of effort / focus.