I would go with any small mountain lake that takes some effort to find. My favorite being rainy ridge because of the countless adventures trying to get there. Catching big guys in Minnewanka or Cold or even ghost on downriggers is also pretty awesome though too.
All depends on what your defination is of Alberta's best trout lake is..
Really that has alot of varations..such as what specie of trout , what size of fish , is solitude a factor.. So it all depends on how you define best..