I'm very weak at leg extensions. Maybe cause I always do them last in my leg workout. But so far, I feel front squats hit the quads the most among all the exercises I've tried including leg press machine, back squat and leg extension. It's been a day since last night's leg training and it's pretty painful for me to get off my chair 

Btw, what happened to your lower back? did you injure it? If you can push so much weight on the leg extensions you should work on strengthening your core and lower back so you can do those big lifts and improve your overall body strength. Just my opinion 

edit: on deadlifts, i  have bad form aka i don't realise that i'm rounding my back. i don't think it's because the weight is too heavy cause my form looks the same even though i'm using warm up weight. guess i'll have to really emphasize on chest out and arched back the next time.