A 6"x24", I would consider a 'must', or even better, 6.5"x24" as well as some square rulers. I have them from 2.5" to 12.5" and use them all the time for squaring blocks. Another ruler that I use a lot is my 6"x12" - I bought this cheap and am surprised at how much I actually use it for small cutting jobs, trimming, taking to class etc.
I have one triangle that I've finally figured out how to use, lol, but can't say it was an investment. One thing that was an investment however, is the June Tailor Shape cut. I use it all the time. In fact, I had DH hand carry it back from Houston to the UK for me as it wouldn't fit in his case!
