Str starts high, rises steadily, and finishes high. However, proficiency in Str saves is only common at a couple of very high CRs. Str's generalized percentage is 56.28%.
Dex starts high, slowly declines, and finishes low. Proficiency in Dex saves becomes common around CR 10. Generalized 66.54%.
Con starts high, rises steadily, and finishes high. In addition, proficiency in Con saves becomes common around CR 9. Generalized 47.01%.
Int starts low, rises slowly, and finishes low. Proficiency in Int saves is only common at five CRs with 12 being the lowest. Generalized 71.02%.
Wis starts moderate, rises slowly, and finishes low. However, Wis saves are common or semi-common at every CR from 8 upward. Generalized 57.06%.
Cha starts low, rises steadily, and finishes moderate. Cha saves are common at every CR from 9 upward. Generalized 54.47%.
AC starts high, rises slowly, and finishes moderate. However, there is no such thing as proficiency in AC. Generalized 67.62%, which I raise to 72.62% when generalizing a spell's damage, because of the chance to crit.
Taking that all together, considering the entire CR spectrum, the preference for targeting should be AC (72.62%), Int (71.02%), Dex (66.54%), Wis (57.06%), Str (56.28%), Cha (54.47%), and finally Con (47.01%).
To break it down into tighter groupings, moving from best attribute to target to worst . . .
At CRs lower than 1, target AC, Int, Cha, Str, Wis, Con, and Dex.
At CRs 1-4, target AC, Int, Cha, Wis, Dex, Con, and Str.
At CRs 5-8, target Int, AC, Cha, Wis, Dex, Con, and Str.
At CRs 9-12, target AC, Dex, Int, Str, Con, Wis, and Cha.
At CRs 13-16, target Dex, Int, AC, Str, Wis, Cha, and Con.
At CRs 17-20, target AC, Dex, Int, Str, Wis, and then Con and Cha in a tie.
At CRs 21-24, target AC, Int, Dex, Wis, Str, Cha, and Con.
Against a tarrasque, if running away isn't an option, target Dex; Int; Wis and Cha tied; Str and Con tied, and AC sort of tied with them. (The tarrasque is unaffected by all ranged spell attacks and has a chance to reflect them back at the caster, so don't even try. It also has advantage on saving throws against spells, legendary resistance, and resistance to nonmagical weapons, among other things, so there aren't really a lot of ways that fighting it doesn't look like suicide.)