Most useful size of circle punches?

3/4" are good for folding in half to make tag/attachment point reinforcers. 1" is probably the most versatile. They get less useful in bigger sizes because it's less likely you're going to need a whole bunch of that circle size in short order. Thin dies tend to cut better, be less expensive, and are more flexible about where you can put them on a piece of cardstock than punches. The best sizes will mostly depend on what techniques they're being used for.

That is how I ended up with two 1/2" punches, lol. I gave one to a friend.