I like the one of the rocks best because to me it's something different; that's the one that caught my attention. It makes good use of the pattern of the rocks, and the ripples of water add some texture to the composition. I might make copies and crop the left side just enough to get rid of the edge of a rock sticking into the picture.
I think the one of the bird is nice and sharp, but for me the one branch sticking straight up seems to cause an odd line in the overall composition. However, looking again I think I might try some crops of that one too; if the left side is cropped some I think it might balance the lines of all the branches better (the bird itself you captured beautifully).
Overall I'd think about how you're using space in your compositions and making sure your focus is where it needs to be. The last waterfall is nicely done but to me seems a little tight in the frame. The flower because of the narrow depth of field seems not sharp (but I think actually the focus is not quite on the center). The first waterfall seems soft but I think actually the focus is on the leaves closer to the camera.
As far as a contest, I haven't done any but have submitted to juried exhibits. I'm finding that the ones that get accepted vary depending on the gallery, the theme, the juror(s), etc. etc.
I just submit my best that fit the guidelines, and some get accepted and some don't. You could try looking at the past contests held by whoever's holding this contest, or maybe look up whoever is judging. I think it can depend too on your 'competition'; how many people enter and how good their photos are probably could affect how well something of yours would do.
And if you didn't already, read the Terms & Conditions of the contest. Many contests online seem to just want submitted photos so they can try to sell/license them, etc.