I've seen all OS, AG, DP and Chronicles episode, and a few BW ones, and I like the OS fillers, including Johto, while the AG ones seem less entertaining to me, and the DP and BW ones bore me. It's probably because I care more about the OS characters (Misty is there, and in OS, I still liked Brock, who went downhill for me in AG and DP, and I already stopped caring about Ash), but also because everything is less game-like. And I just like the Gen I and II Pokemon a lot. I can't really give one clear explanation, but I think the Johto fillers are fine, and better than the AG, DP and BW fillers.