For me, it has been and likely always will be the Kantonian birds. Not only are their designs immaculate and their inclusion paramount to flesh out Kanto, but they're probably the most balanced fun pokemon to use for me with the Flying serving as a pretty good trade-off for the amount of power you receive for seeking them out.
But if I'm being honest, I like birds. And you know what's one of the prevailing aspects of finnish myths? Birds. So it's not a surprise that I would see them as my favorite. It's also not a surprise in the slightest that I would consider making the legendary trio of the region based on Finland birds as well. (The Soul Bird, the Tounelan black swan & the kokko.) But that's not all...
The fourth bird & one of the Fiend-esque encounters will likely be named Sielunel, the Bird of Afterlife. I'll probably make it so it can be battled after catching either the Kantonian or the Kaluain bird trio.
(Sielu+Neljä. Sielu is "Soul" in finnish language, while Neljä is "four" in finnish language. Yksi, kaksi, kolme, neljä.)