After a number of years, I have finally given in, thrown in the towel, surrendered, capitulated. It's not that I ever doubted the consensus view that a high quality, small combination square is an essential tool for woodworkers, it was that I wasn't willing to pay the price of a really good one.
Two years ago, I thought I had outsmarted the marketplace. I took my machinist's square to Sears and methodically went through their 6" combination squares until I found one that was exactly square. I paid my $9 and went home, chortling to myself about what I clever fellow I am. My smugness was crushed by experience for two reasons. The blade was hard to read and it had a tendency to slip. You had to be very careful or the measurement you thought you had set would become a different one. This problem became more and more severe until this fall I couldn't secure the blade at all, both problems leading to highly irritating measurement errors. Exasperated, I threw it away.
I decided to ask for a Starrett, choking as I did so. They're $95. For a 6" combination square! Don't tell me to find a used one. Tried that, couldn't. I have no knowledge of what it takes to make a tool like this, but I really can't understand why they cost this much. I think it may be not only that they are made in the US of very high quality materials but that there is a lot of hand work in the final machining of each square to achieve the level of accuracy they guarantee. I definitely don't think this one will slip. Starrett isn't the only manufacturer of high quality combination squares, but it is the one I am familiar with.
As bad as my Sears square was, it definitely taught me that a small combination square is an essential tool, one that would certainly make my short, short list. It's strength is its versatility. It's the kind of tool that you almost want to carry around in your shop apron.
I have several other small squares. I have the Veritas sliding square and it is better for some applications, particularly when you are making an "x" and "y" measurement at once. I also have this Incra T-rule, very accurate but I almost never use it. In the end, nothing beats a small combination square for all-around utility and accuracy. I could easily live without the others.
I have one more small square that I couldn't live without, a 3" Starrett stainless steel machinist's square that I inherited from my father-in-law. This little thing is so darn handy for doing things like checking an edge when I am jointing, checking my dovetails, etc. I just looked and they cost $70 new.