Anything I use as a ringtone will cause me to hate it, so I use the Homing grenade sound from Borderlands 2. Looking back on that decision, there are more useful sounds to be instinctually aware of, but boy do I bolt awake when a random bandit has a homing grenade.
My notification sounds are the skill level up sound from Borderlands 2 and the Void Reaver denonation sound from Risk of Rain 2 for important things. I've also used the notification sound and train whistle from Rollercoaster Tycoon.
If I were to change my ringtone today, I'd probably go with Run! from Deep Rock Galactic, or one of the battle themes from FTL. Maybe the baseline from a Death by Glamour cover, but I'd fear I'd come to hate that sound, so maybe not.