I think all of them are important, of course, because they all contribute to your overall GPA. The GPA is important and one slack senior year can ruin it. Junior year, like @_Whitetigress said, is pretty important because it’s around that time that you really start thinking about colleges.
And yeah, extracurriculars help a lot and some people say they’re necessary. I graduated with a 4.1 GPA (thanks to honors/AP courses) and had next to no extra-curriculars or volunteer work and I got into my college of choice – a state university, not a community college.
I’d suggest that you look into colleges you’re interested in and see what they’re requirements are. Then you’ll know what you need to work on.