The two favourites I picked are mostly for their role-playing value.
Seal Tribe:
I, also, enjoy how the northern tribes transcend just Finland and can feel very much like northern indigenous cultures throughout the arctic (URW has inspired me to start designing a game in my free-time set in North America's northern plains/parkland/boreal forest, early fur trade era, largely for that reason). The seal tribe is my favourite in that regard because of their mixed food-base (ocean, big game, small game). They are also conveniently located, if using rafts/punts, to make voyages to the south for fancier supplies. I have to admit, I think I also just like playing as far away from the Njerps as possible.
Sarto:
They are good enough in a lot of areas, but I really just enjoy their description from the game. Makes me think of them as very clannish, and I enjoy that from an role-playing perspective.
Honourable Mentions:
Kuikka: It's sad to see they don't have votes yet. I like them for a lot of the same reasons as the Seal Tribe, but I also really enjoy their placement. It's fun travelling a bunch of the world by rivers and big lakes, (unsuccessfully) hunting lake birds and fishing for salmon. They feel so far away from it all in an endless sea of trees, lakes, and bogs.
Reemi: They just have a lot of strengths in areas I make characters for (building skills especially). My most successful characters tend to be Reemi or Kiesse.