I always start quads with squats (I train hams and calves on a separate day), but I may mix other things up a little - like switching the order of hacks and leg presses.
But I do the squats first so that I can do them with proper form, intensity, and concentration. If I do them later, I have to use less weight and less intensity. And I find that my form suffers so that I tend to use more butt and lower back than I need to. I also feel this may make me more susceptible to injury.
But once in a blue moon I may go back and do another 1-2 light high rep sets of squats after doing everything else - starting the workout with heavy squats, then going on to plate-loaded machine squats, leg presses, and hacks. If you are able to squat 500, you may want to go down to 225-275 and do as many reps as you can with good form on this finishing set(s) - once you start to round the back or bow the knees, or otherwise break good form, it's time to rack the weight. Talk about brutal!