Hands down there's no dog I love more than a well structured Border Collie.
Any colour, any coat type, as long as the structure is right and the markings are nice.
Beyond that though....
Working Kelpie
Koolie
Beauceron
Especially Wandering Spirit Vom Wildweg (Haunter)'s structure. Though I think I may like the harlequin pattern better - if for no other reason than I feel it would stave off the "Omg is that a Doberman Rottweiler mix?!"
Dutch Shepherd
And
Staffordshire Bull Terrier
For no other reason than d'Awwww
Overall, I like leggy, lean, pointy, and muscular - but not stocky.
I'm not overly picky on coat type - as long as it's a good "working" type (ie doesn't tend to hold too much dirt & grossness).
I tend to like merle or brindle coats, but a nice Irish spotting pattern in nearly any colour is great as well.