19/16 seemed to be crappy for any type of riding. 17/17 on fairly narrow slicks was so much fun, the bike felt very stable and really rode well. 17/16 would probably be the best for trail riding. 16/16 was awesome for trails, street, and about anything in between, the bike felt less like a chopper and was much easier to ride. anything smaller than 16's I would buy an xr80 swing arm. 16/14 is nice for tighter dirt pit bike tracks, but for trail riding it brings the bike too close to the ground. I never tried 14/14 but I would think it probably rides just as well as 16/14 in tight dirt tracks and would make a great size for street as well. 12/12 Is the way to go for tight street tracks, but I would make sure to go with a 2.50" rim front and rear.
The next XR100 I buy I will run either 14/14 or 12/12 on an XR80 swing arm. 16/16 would have been more fun with a big bore kit and some narrower tires.