My most frequently used are a 4" x 14" (used most) 6" x 24", and my June Tailor Shape Cut rulers. I have several rectangles in other sizes but I actually like the 4" wide one best. I wouldn't mind having the same but a bit longer, like 18". I have squares in a bunch of sizes, but use the 6-1/2" and 12-1/2" most. I've got a few others, triangles and specialty rulers, but don't use them as often as the rest. When I cut a lot of small pieces, I'd rather use a smaller ruler. I guess that's why my 4" x 14" gets used most. It's easier to handle.
Think about the patterns you want to make. For instance I never make huge blocks, so a 18" square would be a waste for me. But if that's what someone else makes frequently, it would probably be helpful for them to have it.
When you do shop, consider the lines on it as well as size. Not just color, but also the amount of lines. I have one that has lines every 1/8" in both directions, and they are all black, it's too easy to cut on the wrong one! I like the Omnigrip with the yellow or green/black. It's easy on my eyes. There are other colors too, so see what is easy on YOUR eyes.
You might want to get something to help prevent slipping too. I like Invisigrip. I have many rulers and have it on all of them, even the June Tailor shape cut - I just cut narrow strips and applied between the slots. I still have leftover from the initial package I bought.