What strollers have the tallest back rests (not including canopy)?
It is usually the larger, often more all-terrain models that only face forward. The Baby Jogger City Mini GT2 or better yet, the Baby Jogger City Elite 2, then also the Britax B-Motion 4 Plus, or the Valco Baby Trend are very spacious. Check also the BOB strollers, and the Mountain Buggy Terrain which has one of the tallest seatbacks for sure.
Btw, there's more to just the height of the backrest - even a taller one on a reversible stroller, where the canopy is right over the end, is MUCH less roomy and won't fit a tall toddler as much as a bit shorter seatback with a lot of space to the top of the canopy. So you will do better to go for a just a bit above average Oyster Zero with a takll toddler, then the Baby Jogger City Premier which was reversible, even if the City Premier has a tall backrest.