I don't follow spin-off scene as much as I used to, sadly. Few years back (say, 2006), I used to be heavily into it, as there were a range of excellent titles available. Now, there are still some things here and there on spin-off side, but none of them seem to appeal to me, sadly.
One of my most favorites, however, would likely have to be Gale of Darkness. Although I did not get to finish it completely (borrowed it from a friend, and she took it away from me before I were able), I had still enjoyed my time on it; things were rather enjoyable! They've had shadow-Pokémon, which I found to be quite an unique concept; it was very fun to be experimenting with them.
I would like them to remake this game at some point in future, if possible. If they do, I'm sure many of us will enjoy. I wouldn't mind if it were for Wii U, but I feel it would be slightly better if it were remade for 3DS instead; this way, there wouldn't be much of hassle when connecting main-series games to it (sort of like how it was in generation three), if they're planning to re-implement the system.