I always use a bowline. will take all you can put on it and undo it without dramas. if you have a tail left on the rope when your finished, a couple of half hitches around the rope finishes it off nicely.
I religiously learned to tie a bow line when I read it was "the" knot to use for tying down... It is fantastic on one end, either on the aircraft or on the ground, however i've found its pretty useless tying a bowline on both ends, as its next to impossible to get decent tension on the rope once one end is already tied down..
I'vealso tried using a bowline with two half hitches on the other end, and while it is secure, again, I always end up with too much slack in the rope, and you have to spend way to long stuffing around with it to try and get it taught.
I have some mates who can tie a "truckers hitch" - and this one seems to be by far the best i've seen. You can easily adjust the tension on it once the knot is in place - and any pulling force on either end only tightens the knot further. Only problem is I havnt been able to master tying it by myself yet - it is not the easiest knot to learn.