Masterpiece and Alternators. Besides those two: RID, Beast Wars, and Cybertron

 

The biggest thing I have (personally) about this whole subject is what I might consider a highpoint in design or engineering is not necessarily what I really like in a TF toy. (and it's a highly subjective subject to start with)

I tend to prefer simplicity over complexity, for example (just enough to do all modes effectively. I dislike complexity for its own sake) and find most mainline toys more satisfying and enjoyable than the majority of collector-oriented stuff like Masterpiece and Alternator lines

 

Beast Wars Neo was the epitome of the BW lineup. Compare the stegosaurus Saberback with any other BW deluxes, and the engineering marvels really stand out. It really played with the whole "let's not have a beast head as a chest" syndrome, and worked out marvelously.

Car Robots / RID was also a great line. We got some of the most complex deluxe-sized cars, and also engineering marvel that was Omega Prime.

 

On a completely different level. Classics is the first line to come along in a LONG time that I am completely satisfied with.