One of the prettiest quilts I ever saw was a simple 9-patch using white muslin as the light squares and the plain squares separating the 9-patch blocks. Every block seemed to be different because the maker was determined to blast through her stash the fastest way possible - the 9-patch blocks were assembly line fast and what could be simpler (or cheaper) than white muslin to offset the colors?
My husband's most favorite quilt that I've made for him is a simple 16-patch of medium & dark Civil War reproduction fabrics paired with cream or light beige Civil War repro fabrics. All the blocks go together beautifully. After seeing that first quilt and making this second one, I've never underestimates simple squares and simple 9- and 16-patch quilts ever again!
THAT SAID, if you want to try your hand at rectangles, my favorite pattern is Yellow Brick Road - it can be done with any color palette, the layout is very forgiving, and the size of the units is very comfortable to work with for beginners. I've made every size offered in the pattern at least four times.
Good luck and stay encouraged!