It kind of depends what you are after!
I have the first three but not the Morrison et al book.
Gill is the best all rounder. I have the 3rd edition but I don't imagine the 4th is significantly different.
It has six sections covering 'origins' (systematics), 'form & function' (anatomy broadly), 'behaviour and communication' (senses and sounds), 'behaviour and the environment (socialisation, migration etc), 'avian life histories' (reproduction) and 'population dynamics and conservation'.
It's a decent sized book - just short of 700 pages of content, plus a big section for references.
It's very thorough and I found it easy to read and use.
I'd describe Scott as 'Gill lite'! It has a very similar content, but just 150 or so pages, so obviously the depth will be much less.
Proctor and Lynch has a narrower scope - as the blurb says ' a visual guide to the structure and anatomy of birds', which it covers in as much detail as the ornithologist (as opposed to a vet!) would most likely need. It includes some useful content on field techniques (measurements etc.) It does cover some more general topics very briefly. It has just under 300 pages.