Finishing the physics graduate curriculum as undergrad I am a year ahead of my physics curriculum, but I plan to stay as an undergrad for the 4th year to increase my chances at top graduate schools in physics. In this 4th year, I will take all the graduate courses in physics a typical 1st/2nd year PhD student would take: two semesters of quantum mechanics and a semester each of statistical physics and electrodynamics.

My question is: is it worth it taking all of these graduate courses as an undergrad? From what I know, most universities don't accept transfers in graduate courses and I would have to take them again.

I would still like to take them because I am eager to learn them, and also achieving good grades in them would help me demonstrate my readiness for graduate school.

Other option would be taking courses in other disciplines that would benefit me, such as math or electrical engineering.
Research experience is almost always better than additional classes, even graduate classes. Either graduating early so that you can work/volunteer full time in a research lab or taking the minimum number of credits to maintain your status as a student and work/volunteer part time in a research lab is likely going to increase your chances of admissions to a top graduate school much more than taking a full load of classes.

If you go with taking classes, classes that fulfil general graduate requirements are going to be less valuable than classes that will help prepare you for your research.