It depends on what you're looking for.
Broadly speaking, there are two types of Behavioral Neurology and Neuropsychiatry (BNNP) fellowships: those that see memory disorders / dementia and nothing else, and those that see what I would call the full spectrum of BNNP. There isn't a right or a wrong answer as to what type of program you should look for. If you goal is to see dementia, run a memory disorders clinic, or do research predominantly in neurodegenerative disease, then the first type of program may be a perfect fit for you. I chose to pursue fellowship training at the second type of program, and my clinical mix is about probably about 75% dementia and traumatic brain injury (mostly mild) with cognitive and neuropsychiatric sequellae. The remainder is made up of functional neurologic disorders, primary psychiatric disorders with cognitive complaints, etc. My residency training convinced me that I wanted to be exposed to the breadth of BNNP training.
As far as programs to look at:
Directory -- United Council for Neurologic Subspecialties
Programs that, to the best of my knowledge, are primarily dementia / neurodegenerative focused: WashU (though technically not a behavioral fellowship, I believe they have a post-doc / research fellowship focused on AD), Mayo, Columbia, UCSF (heavy FTD focus), OHSU, Yale, Northwestern (Mesulam is the division chair, strong PPA / language focus)
Programs that are broader: Brigham and Women, MGH, Hopkins, UMass, Colorado
Some other programs to know: UCLA (Mendez is prolific and has published something on just about every topic you can think of), Florida (Heilmann is there, but is older so question of retirement?)
IMO, it's important to figure out what you want your fellowship to prepare you for. I like seeing post-concussive symptoms / mTBI, so I chose a program that would give me exposure. Your ideal program may include never having to see that. Feel free to PM me if you have any specific questions and I'm happy to try and answer what I can.
