I would say the last is the best, simply because it has the better focus, and the other bird in the background adds some interest, even thought the twig in front of the first bird is a little obtrusive- I know you could have helped that but in the future keep an eye out for those little things and try to angle and shoot around them
Some things that you could improve on would be your focus. All most all of these are blurry and out of focus. Possibly because you greatly cropped them, you had some camera shake, or just didn't have a sharp focus on your subject to begin with. Try a few with a tripod if you have one, that might solve some camera shake.
Also to improve a few of these and make them sharper you could use a post processing program like "gimp" (its free) and use a unsharp mask. Don't go overboard, but a little sharpening would greatly improve these. A color boost and a contrast boost might help too.
And on the second and third, watch your composition- the birds legs and tail is chopped off quite a bit.
I hope this helps! Good luck with future bird shots