I agree, Kiril would be my first pick from what I see. He’ll be the most versatile for all titans, Rare Quests, and events, which are all good sources of additional ascension mats for other heroes. The combination of attack and defense buff, plus healing, is highly useful.
Another vote for Kiril. You’ll use his attack and defense buff everywhere, and you definitely need a healer in that lineup. After him, I’d do Hansel for the fast mana control. Then, if you pull Wu, I’d do him as your yellow- or else Chao works too for the fast mana cut.
Then you’ll have your first full rainbow team of maxed 4*, and can start on maxing Grimm for the best pulverizer, Sabina for a deep immediate heal and dispel, Triton for a fast sniper that’ll increase your healing received, and Kashhrek if you find yourself needing a solid defense and war tank.
If you finish all of them before you pull any new 4*, you could finish those 3* for events and filling out your roster for war and color-stacking. Brienne and Bane are both very good 3*. I’d also keep Valen, Ulmer, Gato, Gunnar, Nashgar, Hawkmoon, Azar, Rudolph, Berden, Belith, Mnesseus, Namahage, Gan Ju, Kailani, Melia, Balthazar, Tyrum, Gill-Ra and Chochin too- if you happen to pull them.