The last "new", as in one that opened the most recent, was Gringotts for me, but if we're not counting that, it would be Flug der Dämonen at Heide Park.
Otherwise, the last new, as in my first ride on it ever, was also Gringotts (#123), and if we're not counting it, Mystery Mine at Dollywood (#122).
This summer, I'll be riding Fury 325 (I'll try to make that my 125th credit, with a 124th either being a family ride at Carowinds, like Flying Ace Aerial Chase or Ricochet, or the Seven Dwarf's Mine Train at Magic Kingdom)!