This is just a damned popularity contest!
Orion... Mainly because it is the 1st constellation I learned to recognise, long before i actually got "into" astronomy.... Having since discevered that it contains the great orion nebula AND the horsehead nebula doesn't hurt either
Altho, having only recently started learning many of the other constellations I have to say I have really grown to love scorpio, mainly because I can so easily recognise the scorpion in it! I find it the easiest constellation to associate with what it is supposed to represent, by far