Longest shot was 70 yards on a running muley near Moab, UT. Mostly luck, since I was following him with my scope through some heavy scrub oak running full speed perpendicular to me, waiting for him to cross a clearing. The gun went off unexpectedly (I was aiming where I thought he would be, but couldn't see him). Found him lying 10 feet beyond where I shot, drilled through the neck. Only a 3x3, but with a tall 25" spread. (Winchester model 70 XTR in .300 win mag)
Longest shot can be relative. When I was in high school, I used to hike around with a Jennings J-22 pistol (one of those cheapo pocket .22s) that I bought for 40 bucks. I hit a prairie dog at about 25 yards, a snowshoe hare at 20 yards, and a grouse (in the head) at about 15 yards.
However, my best day of shooting ever was jackrabbit hunting in Wyoming from a jeep with 3 of my buddies. Got 8 rabbits with my Sig p226 9mm. The other guys got 1 each. The best part was that at the end of the day, they had each fired several hundred rounds. I still had 15 left from a single box of 50 hollowpoints. My shortest shot that day was 25 yards, longest about 40 yards. All 8 rabbits were running full speed when hit. Serious fun!
I've always been decent at handgunning, especially with moving targets, but my long range rifle skills have never been all that great. I'm hoping to improve my skills over the next couple of years with lots of practice!