With squat I need a partner, with the sled I can work alone. Tend to like doing high rep work on the sled, but I'm becoming an old fart who's losing his mojo, so you need to take that into consideration.
I like hack to help increase ham strength, before I increase my squat strength. Always felt the more important muscles were in the back of the leg. Squat is the basic though. Its the meat and potatoes of training. The sled, I use as a recovery tool to help everything, more accessory I guess.
squat is #1 best / most effective training for overall conditioning and muscle building imo. Nothing is better. Unfortunately after a herniated disk squatting is out for me. Leg press would be # 2 and hack last.
For a long time Leg press had been my first leg exercise, now I'm going back to squat first, then leg press or hack squat. I'm not a fan of hack squat for the same reason I avoid standing calf raise, the weight compressing down on your body makes it hard to breath which makes it hard for me to think about doing more reps and pushing when I can't breath.