After reading the posts, I would say all of the above except the go baby. I have had the studio (heavy, takes quite a bit of space, more expensive dies). I love my little go baby for special items like flowers, circles, robbing peter to pay paul (can't think of the other name). Basic squares and half square triangles etc. I use those to build my stash precuts.
I think my vote goes for speciality rulers. The only specialty ruler I use ALL the time is the salem one for perfect right angles It folds up and has long legs to put along the fold line for getting a perfect straight edge. Sorry I don't remember the name of it. Costs around $30 and worth every penny.