Animals is a shining beacon for Roger Waters, the man. Roger got to fully express himself without much interference from Gilmour or Wright. What I mean by 'express himself' is, he let all his anger come through on Animals where if Wright or Gilmour were more involved, we may not have gotten such a politically charged album. Roger is fantastic at lyrics, but I think Gilmour & Wright had a better feel for the music and melody of a song. That being said, Animals has never been any where close to a favorite album from PF.
I'm not going to choose Pigs because it sounds too much like 'Have a Cigar'. It is the most accessible of the 3 songs.
Sheep has the same echoing effects as 'Run Like Hell' and has a lot of Pink Floyd atmosphere, but I am not that fond of this song. If you go back to some concert footage from 1974, you would hear 'Sheep' played as "Raving and Drooling".
I'm going to go with Dogs. This song was also written in 1974 by Waters & Gilmour which was quite some time before the idea to put Animals together. and had been played in concert as "You've Got to be Crazy". This song has some of Gilmour's best work and it is the most complex song on Animals......it gets my vote.
You've Got to be Crazy