Right now I only have 8in circle, and 6in circle pans and I was wondering what else I should invest in. I like making unique things so I was thinking about the wonder mold. A sheet cake pan might be nice too. What pans do you use the most, just for casual baking for friends and family? What pans will I get the most use out of? Thanks for your help in advance!

I use mainly the 8-9 inch circle, 6 inch circle, 9x13 rectangle (AKA 1/4 sheet), wondermold, mini wondermold, and cupcake pans. You can use an oven safe bowl instead of the wondermold.
I have a 10 inch round that takes 2 cake mixes, and I'm using that more and more. This whole big cake all in one pan, is appealing to me.

For me, I just bought pans as I needed them. There is so much cake stuff out there that you could really go crazy! I just limit myself to whatever I need to finish what I am working on. Saves money!

I also bought stuff as I needed. I have a 10, 8, 6, 1/4 sheet, half sheet, a couple of dome pans a star pan and a winnie the pooh I used for my neice. Some springform pans for cheescakes and a couple of cupcake pans. Since I do cakes for my family and friends and we are alot of people, I do use them all regularly. Next I have my eye on the pans that look like paislies. So pretty...

It depends on who/what you are baking for. A sheet pan would give you some varity.
BTW: if you match the cake pan size to the board & box sold as a 1/4 sheet you will find a 12x8 is a '1/4 sheet' not a 9x13.