Kiril for versatility and stacking with Grimm. My Kiril + Grimm combo allows my hitters (most often Lepus or Seshat, or Triton in 4* tourneys) hit really hard. The defense buff is great too, and allows my heroes to survive hits they may not otherwise.
Triton has an interesting healing buff, but is a lot more niche (though Kiril + Triton do make an excellent combo, you could consider Triton as your next project after Kiril). With Kiril/Grimm/Trition, you can choose to use Triton to boost Kiril’s healing; or use Kiril+Grimm first to make Triton hit extra hard.
Sonya is less urgent to max because you have Caedmon already. Though you should still max her eventually, to bring against Reds for example.
Boril has a shorter “shelf life” in the long run. Best on defense and against strong bosses/enemies, but Kiril > Boril even there. Boril makes the enemies kill themselves; Kiril keeps you alive longer so you can kill the enemies proactively.