Once again, it's an interesting subject.
My collection is mainly oriented by the G1 toyline.
In the past I have also bought post-G1 toys (G2, RID, Armada...) but finally I have realised that I always looked on these modern toy line the original G1 robots.
So I have resold a part of them (or store them away to my display room) and I have recentred my collection to the G1 toy line.
My quick point of view about :
Masterpieces : very cool robots for most of them, but I never bought any MP, maybe because they are not really "toys", I don't know ^^
3rd party : Many are nice, but the choice is now enormous, maybe too enormous and the place in my display room is limited, son I stay focused on the real TF line.
Classics : good idea with some good robots for a G1 fan (Hot Rod, Cyclonus, Hound...), but some are too far of the G1 representations (Power Glide..).
Combiner Wars : also good idea with some good robots, but the "combiner" fonction is often useless without any interest and contribues to make the robot ugly (the knees of Bombshell are a perfect exemples).