On pure vocal talent Mercury by a country mile. Plant and Jagger were good enough at singing but even better as songwriting partners IMHO. For me the Stones and Led Zeppelin were more about the music, and while Queen had some excellent musicians as well, Freddie Mercury songs were mostly about the singing.
Freddie – never saw an artist have such a huge connection with a live audience – had an amazing near 5 octave range, ranging from a bass’ F2 to a soprano’s F6 (which apparently kept increasing with the years passing), but what was most impressive was his ability to jump from one end to the other effortlessly.