Beast Wars and Beast Machines are still the two best series in the franchises 25 year history. While vehicles tend to be much cooler, BW/BM had a great plot overall and was a very good show as a result. It's a shame that finding the DVD sets is now very difficult. It's probably the reason Transformers is still around too and is as popular as it is (other than the movie). If the franchise didn't do well in the 90's with BW, who knows what would have happened to Transformers. Chances are, it would have come back but we probably wouldn't have a second movie coming out right now.
Obviously, the current incarnation, Transformers: Animated, can be watched on Cartoon Network so it is the easiest to get your child into. Just go out and get some of the toys and see what happens.