I collected some Beast Wars when I was younger. I was enchanted with the Transmetals, in particular Optimus Primal and Megatron, which I proudly owned. I started to get tired of always having to keep up with the updates (I remember when I looked at Optimal Optimus for the first time at the shop, I was like, WTF? Not getting that), and when I saw Transmetal dragon Megatron, rather than feeling excitement, I just didn't feel like buying him. So ended my Beast Wars fun. But Beast Wars should get 7/10 overall, given their creativity and courage in trying something new.
I tentatively bought Mega Cheetor in Beast Machines and after a week I never looked at it again. No fun, no interesting design, blargh. I dropped out of Transformers altogether. Beast Machines is probably the worst series I've personally experienced. 2/10
In terms of play value I would have to say Energon and Cybertron are the best. From what I've seen last year and before, they had a massive range of toys, and massive figures like Starscream, Primus, Galaxy Convoy, and heaps of neat little gimmicks that I personally was attracted to (although I never bought any). But this was when I remembered that most Transformers aren't as poopy as Mega Cheetor. 9/10
I think the movie line has been pretty good; it has reawakened my fandom which essentially died out after I bought Mega Cheetor. The movie toys offer reasonable play value, have a reasonable range of Autobots and Decepticons, and the Scout Class ones are pretty cool. The new designs I welcome, in particular Cybertronian Protoform Prime and Megatron (although Megatron could have been executed much better. In fact, the new aesthetic with the movie Transformers is what I like. G1 fans may hate it, but as one who has just gotten back into Transformers, the 2007 line is perfect for a fresh start to both the toys and the story canon.
Did I mention I like Prime's new look, both in his Protoform and Earth form?

So overall personally I think the movie line is the best, although Energon and Cybertron are probably more popular with the kids and I am not surprised at all.
So IMO the best series are Energon, Cybertron, and 2007.