I have a lot of rulers bought at Joanns with coupons or on sale.
The very basics I use the most are the 6 1/2 by 24 and the 6 1/2 by 12.
But for special rulers my Quilt in a Day flying geese and also the 6 in triangle sq up ruler are my best investment.
I also like to use my largest 16 in square for squaring up my quilts.