If you want to go by the expanded Japanese timeline, then you should watch in the following order:
Transformers (US season 1-2)
Scramble City (Japan OVA)
Transformers the Movie
2010 (US season 3)
Headmasters (Japan-only)
Masterforce (Japan-only)
Victory (Japan-only)
Zone (Japan OVA)
Beast Wars (US season 1)
Beast Wars II (Japan-only)
Beast Wars II the Movie (Japan-only)
Beast Wars Neo (Japan-only)
Beast Wars Metals (US season 2-3)
Beast Wars Returns (Beast Machines)
Headmasters is currently available subtitled, though only released in Europe. The good news is that the DVDs are region-free, so regardless of where you live your DVD player should play them. Masterforce is coming out in July and last time I talked to Chris McFeely, he said that Victory was just about finishing up and should be out before the end of the year. He also said that Metrodome has plans to release the two OVAs, Scramble City and Zone, on the 20th Anniversary UK DVD of Transformers the Movie this year.
No word on any of the Japanese Beast Wars stuff getting DVD releases.
Now, if you want the simplified US timeline...
Transformers (season 1-2)
Transformers the Movie
Transformers season 3
Transformers season 4 (The Rebirth)
Beast Wars
Beast Machines
It's less complex, but the Japanese universe has more material within it. So it's really up to preference.