Syd Barrett was the leader of a group that happened to record one of the best psychedelic albums of the 60's. They also happened to be called (the)Pink Floyd.
But for me (and I assume many others) it didn't really "Become" Pink Floyd until Gilmour took the stage. To me, Gilmour IS Floyd. It's his voice and his guitar playing that are the signature sound of the band. Of course I'm not dis-crediting Waters, Mason or Wright for their input...it's just that Gilmour is the pulse (no pun intended).
When the group started going downhill (Animals, The Wall, The Final Cut) it was due to Waters taking too much control and not letting the rest of the group contribute. Not surprisingly, it's the Gilmour written tunes on Animals and The Wall that still sound good to me.
Roger Waters - Just Another Prick In The Wall