All by taste.
I enjoy gestalts alot and tend toward those, as well as headmasters and targetmasters.
The new ones all depends on style and feel of a series. I didn't have any beastwars until the transmetals, which i had like all of. Had only the vehicles (vehicons) from beast machines, almost all of RiD, all of aramada. Zero Energon and beyond. I like the strictly vehicle/G1 theme series.
I do not buy repaints unless there is a significant difference. I must have basically every Prime/Megatron. Bought almost all the classics, wanted the alternators but they were too much money and the transformations were too similar. MP's are must haves except stupid repaints.
Go with what I like, unless i really like a certain line and then i buy nearly all of them. There has been lulls in my purchasing because of this and during those times I may fill in the gaps with some reissues or G1's I've wanted forever. PREDAKING