Hi. I'm no expert whatsoever on this activity, but my fav is #3 as its captured nicely, well exposed, no clutter in the bg. #1 almost does it, but those blessed advertisements! The others (for me) are more like snapshots, but the timing looks good. Only ever been to one point 2 point, hardest thing ever to get a good position to shoot the action from!
No 4 by a country mile - it's an excellent jump shot. 6 is a great shot but slightly marred by the control tower; sometimes there's just nothing that you can do about that however, although you could open the aperture a bit to /3.5 or so to knock the background out a touch more whilst keeping the horses head and rider in.
Your horizons need sorting on 1,3 and 4.
I realise I'm still not getting it quite right!
Having a wonky horizon is caused by not holding your camera level when hitting the shutter release, or clicking too hard and titling to the right, which then leaves you with skewed images.
It's fairly easy to fix in most PP software and the simplest way is to look for something that should be vertical and adjust it so that it is. Horizons themselves often aren't level due to perspective or ground shape, but it's a generic term for your pic slanting to one side!