I'd look at which of the four class-groupings are good at each of the five saves and in which abilities those classes should have good scores. In the PHB (1978), aside from the minimum requirement of 9 in the prime requisite for each class (Wis, Str, Int, Dex), it says clerics benefit from a high Str and Con, and (perhaps secondarily) from Dex. Fighters also have a minimum requirement of 7 in Con and benefit from a high Dex, while magic-users have a minimum requirement of 6 in Dex and thieves are said to benefit from a high Int. So we have the following minimum ability scores associated with each class in order of importance:
- Clerics - WIS (9), Str (5), Con (5), Dex (-) / not important: Int (5), Cha (5)
- Fighters - STR (9), CON (7), Dex (5) / not important: Int (-), Wis (5), Cha (5)
- Magic-Users (including Illusionists) - INT (9), DEX (6) / not important: Str (-), Wis (5), Con (-), Cha (5)
- Thieves (including Assassins) - DEX (9), Int (5) / not important: Str (5), Wis (-), Con (5), Cha (-)
Paralyzation, Poison, or Death Magic: Clerics generally have the best save against this, but fighters are good too (except at the earliest levels) and share the top spot with clerics from levels 13 to 18. For this, I would go with your best of either
WIS or CON.
Petrification or Polymorph: Fighters are best at this from levels 9 and up. Thieves are best for levels 1 through 6 but are consistently among the worst starting from level 11. I think the best way to do this would be to use an average of your
WIS and DEX for lower level challenges and then to transition around 9th level to using
CON or an average of
CON and CHA.
Rod, Staff, or Wand: Early on, magic-users are the best at this, and fighters are the worst, so it should be a straight
INT save. Then at level 13, fighters catch up to magic-users and share the top spot from then on, so higher level challenges should use an average of
STR and INT.
Breath Weapon: This follows a similar pattern at lower levels, with magic-users having the best save, so it should be a straight
INT save. At level 7, fighters begin taking the lead and thieves start falling behind, so challenges at that level or above should use the better of either
STR or an average of your
STR and INT.
Spell: This one's nearly identical to "Rod, Staff, or Wand" above so should use the same saves.