I bought the Hexagon Die to cut hexies for English Paper Piecing primarily but you can use it to make a machine-sewn quilt as well.
I heard they are great because you can make HSTs and flying geese without having to square up or trim tips.
Another die that I got bundled with my cutter purchase was the value die which has a 4 1/2″ square as well as a 2″ square and an HST cut all on the same board.
The 2” Uppercase Alphabet Die is another one of my most used dies. I use it for putting kid’s names on things mostly.
I have a number of Strip Cutter Dies for cutting fabric strips and I love them. The one I use most often is the 2.5″ Strip Die which I use for cutting my binding strips.