Sissy squats are the best exercise to isolate the teardrop muscle in the quadricep but if your quads suck - you need more overall mass. You need to focus on your bread and butter exercises like squats, leg press, lunges etc. Not cutesy exercises like sissy squats.
Look at the guys with the best legs at the gym. They will be squatting heavy weight for reps. They won't be doing cutesy exercises like sissy squats and glute kick backs.