Martelli rotary mat...the quality makes it a joy to use. Because there are ball bearings in the movement, I can safely cut pieces and smoothly turn the mat to make another cut. I don't play games with rotary cutters and do odd body twists to get a cut.
The best investment thus far has been my Bernina 765 and a Horn table with electric lift to set it on...there are so many bells and whistles on the Bernina and I don't know how I've lived without them...plus, the electric lift on the table has made positioning my machine so easy and so accurate. I don't regret a single penny spent on those two items.