Depends on the game
If it's generally an easy game, then I'll often hoard until I hit a difficult section, or possibly just forget to use them entirely.
When using them, I'll generally favor attack boosts for the character that's attacking the most, similar thing with magic attack. I also indirectly favor the main character, since they're usually attacking the most.
Speed boosts will probably go to my strongest attacker, so they get more attacks.
For defensive boosts, I'm more inclined to cover weaknesses so that everyone can stay alive. Probably will favor the healer, particularly if the healer is the only one who can revive. If it's a strategy game, where I'll probably give it to the characters who are almost good tanks but not quite, rather than the best tanks who are probably doing just fine staying alive already. If it's a game where main character death is game over, but other characters are allowed to die with little consequence, I'll definitely favor the main character here.
Within all of these, I usually don't dump them all on one character but spread it around a bit. Just the ones I mentioned usually get first priority.
"so is my word...It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it." - Isaiah 55:11