Poison. AD&D saves have a hierarchy- the first applicable save on the list is the one you use.AD&D saves made no damn sense. If I get hit with a poison spell from a staff, what do I roll against?
F/R/W made sense and it was easy to figure out what you would roll against. We don't need forced grid symmetry of 6 stats 6 saves.
Amusingly, this should mean that "Spells" is the last port of call, but I remember making more Spell saves than anything, largely because a lot of people didn't read the section where this is explained.
There's a few more caveats to saves as well that most people missed, like how your Dexterity defense adjustment applies to saves against things you could theoretically dodge (I assume this is why the Thief has such terrible Breath Weapon saves- since you're heavily incentivized to have high Dexterity), Wisdom applies to anything mind affecting, and so on.
2e added an interesting wrinkle most people missed, that magic armor can add to many saving throws (save for things like immersion in acid), which leads to one of the big issues with AD&D saving throws.
It's too easy to get very good at them. Not only do you get better at saves as you level, but bonuses to saving throws were ubiquitous from ability scores, races (the infamous "shorty bonus" for Dwarves, Gnomes, and Halflings), class features, spells, and magic items. The entire point of saving throws could get left behind in the dust as the game progressed- there was even a magic item that let you make saving throws against things that didn't allow saving throws!*
*(The Scarab of Protection)
Now granted, the fact that so many saving throws were lethal if failed in AD&D probably makes this sort of thing necessary, but it gets very bizarre when the most terrifying things in the game have only a 5% chance of affecting a character.