Outside of the "get 1 of every size" approach, do you have a particular length of rule, t-square, square, combination square, etc. that you tend to find yourself using the most?
When I don't need to use a guide rail, the 6" Swanson Speed Square is the one I use most frequently off the MFT. On the MFT, it's the Woodpeckers 450mm Precision Carpenters Triangle. Then there's the 3m Talmeter tape measure.
I have LOTS of traditional machinist squares from years of being in that industry. They are mostly poor for woodworking. I do use the 4" double squares, and sometimes a 6" double square a lot. These are used as often for making a depth gauge as for a square. If you want a repeatable line at 1/2" just move the square to that point and leave it there. I actually have 4 4" double squares and 2 6" and wouldn't want to do with much fewer.
For a traditional try square with the bonus of markings, I have a Woodpeckers 1281 square that is heavily used. It is vastly superior to the traditional machinist square in the same sizes but the Woodpeckers is less flexible so sometime I do resort to the 4 piece machinist square. With drawers full of various squares, probably 20 in all, I probably use the 1281 and a couple 4" double squares as much as all the rest put together.
Really happy that I invested in the Woodpeckers 32" T-Square. Great for cutting up sheet goods, setting down a straight marker when domino-ing a wide panel, and I also use it (or a line that it created) on a scrap board to quickly square my MFT.
Rockler sells a relatively cheap assortment set of engineer's square, which I use for marking edges, short boards and squaring up my stationary power tools: