Well, it depends on what you mean. For practicality, I'd say the best one I have is the Kensington Halloway, it's a shoulder bag (few LAMBs are!), is a good size, has lots of pockets, and is a style/color/print that can go through the seasons!
(picture is dark, sorry)
If you mean like quintessential LAMB, I'd say anything in the Saddle, Rasta, and Silver prints are must-haves!
Ughh so hard to choose! For everyday use I love the Dorset Covey, I have it in black so it goes with everything, it has 3 sections and it's HUGE so you can lug around everything you need. It has the option of 2 straps so you can wear it on your arm or over your shoulder (although the leather handles fit comfily over my shoulder too) - handy for keeping your hands free for shopping etc. Did I mention that it's huge??
But for a pop of color and really original style I agree with you girls, you can't really go past a Mandeville in Rasta!
I am a shoulder-bag-only chick, so I would say, for me, it's the LAMB Oxford. I have one in silver & cheetah and love them both! It is heavy, but it has 3 separate zipper compartments and keeps me super organized! :)