Best contract next season or best contract in four years? Those contracts are going to leapfrog and with the cap increase coming, Favors' next contract might be huge and make Kanter's deal seem like a bargain, even if we "overpay" in order to match.
Right now we have no idea how much Kanter and Favors will improve. So you keep all three. Gobert can get 30 mins off the bench in a 3-man rotation. And then assess as you go. Maybe Kanter eventually becomes a 6th man so we can have his scoring with the 2nd unit. Booker is traded because his minutes are gone next season.
Outside of Towns/Okafor, which is so remote a chance it's not even worth considering, I think the Jazz still need to go after a stretch-4 or a wing in the draft. I like Porzingis or Johnson. Then do the reverse in free agency: get a wing or stretch-4. There is always roster turnover. Better start planning now for Favors, Hayward or both to leave in 3-4 years.
BTW, if we extend a QO to Kanter, it's unlikely we could free up enough cap room to go after a free agent like Millsap or Matthews.