It obviously varies by the game.
That said, as someone who actually makes a point to try to understand damage formulas from as many games as possible, the answer is weapons a shockingly large amount of the time. Defence often just isn't a very potent stat mechanically. Furthermore, as already noted, armour often doesn't affect damage dealt by magic, and in some games bosses favour that.
The most egregious example is cases like Final Fantasy 7, where literally the difference between, say, the starting armour and the Crystal Bangle (final storebought armour) is a cut of about 12% physical damage received... so you can imagine how tiny any given upgrade is. (Armour upgrades are notable for materia slots and elemental properties, but not defences.) That's on the low end for worthwhile defensive stats, but not by as much as you might imagine.
Weapons, meanwhile, are almost always a meaningful upgrade, except for characters who don't use them to deal damage. (Granted, in some games, like Final Fantasy 6, that can be a lot of the cast!)
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