Best three for me were probably the original penguin (and I like the latest limited edition), the Triton (I think) kit of the 1994-95 season, the bue-blue-white of the late 60's and the first Premier League it.
Dishonourable mentions to the dolly mixture kit, the Karanka-era thing with the white sleeves and the vicar's collar shirt of the 1995-96 shirt,
In passing, I wish we'd stick to a theme (with variations, of course, to sell more kits) - the 'big' clubs tend to and we're on the way there too.