I picked Titans Return partly for sentimental reasons. Highbrow was the first Autobot I ever received as a child, and was the second or third Transformer I got after Pounce and Wingspan. Also, the first Transformers fiction I ever consumed were reruns of the Headmasters anime when I was living in KL in the early 1990s. So the Titans Return line strongly represented the time that got me hooked on colourful plastic robots.
Another thing I liked about Titans Return was the amount of variety offered in terms of alt-modes and characters. Not to put Combiner Wars down (which is also a great line), but with the exception of several Legends figures like Shockwave and Bombshell, most of the Combiner Wars line-up transformed into Earth-based Vehicles. With Titans Return on the other hand, we had futuristic/alien vehicles, beast modes and triple changers. As for character selection, not only did we get the original 7 G1 Headmasters, but they threw in the Decepticon Targetmasters, the Jumpstarters, Black Shadow, Quake and Krok (seriously, how often does HasTak offer a new Krok at mass retail??). My only complaint about Titans Return is that I would have preferred characters like Blurr and Octane to be regular Transformers than Headmasters, but I can understand why they went with that direction.
As mentioned earlier, Combiner Wars is a great line, but I just prefer Titans Return better. I also feel like TR Deluxes and Voyagers make better standalone toys. My own personal quibble about CW/PotP Deluxes and Voyagers is that to exploit their gimmick and enjoy the figure to the fullest, you can't just buy one, you need at least four Deluxes and a Voyager. Still, I really do appreciate HasTak offering modernised versions of the Gestalts with better articulation and proportions.
As for PotP, my favourite aspects were the new Dinobots and Leader class figures. It’s so cool to have an Optimus Prime with a Matrix, and the option to turn him back into Orion Pax.