When I first discovered Gierach, I read every book he wrote except for the bamboo book in a span of 2 months. I believe he talks about Labrador many times. I could've sworn he claims a 9-10lb brookie in a 20ft stream. That would be nuts! However, I don't want a brookie. I've caught plenty of them, mostly because I live near enough to chase 'em.
.....wow...this is a tough one...
The 5 lb Gill sounds awesome but I don't think that's a proper answer to this question.
My generic answer is going to have to be a Golden Trout (or maybe Grayling), a real one, not one of those stupid genetically disturbed "Goldens" that can be caught in the East. Catching a Barracuda and watching it shoot out of the water like a rocket while attached to my fly rod would be awesome too.
If I had little time to fish however, I'd want more than anything to go down to the creek I grew up on and teach my boy how to catch the biggest Redeyes you've ever seen out of a tree like I learned to. By then, he may already know, in which case, we'd just go catchin' em 'til I get called home.