• Never keeping beer in the fridge (bad habit taken up during lockdown era) so drinking at home doesn't become a normalized ritual because it's there.
• intermittent fasting: indirectly it's made me more energetic and focused in the morning and early afternoon. It's made me comfortable with cravings and slight hunger, not reaching for a snack or food instinctively. Made me appreciate eating, and eating higher quality food that nourishes and satiates.
  Bonus: Save money on less groceries/snacks. Don't gain weight as easily.
  Don't even need to be extreme about it, can skip days and not a party pooper if going to an early breakfast or late dinner. It's about the trend not the exception.
• Having a regular sleep/wake schedule: this was a huge one.
  I was on a rotating schedule, early morning, afternoons, evening and night shifts, for a big part of my 20s. At that age, especially early on, you feel invincible. You can sleep little, pull all nighters like it's nothing. Eventually it takes its toll and swapping to a regular 9-5 schedule has been hugely beneficial. The circadian rhythm hype is real, as is the quantity of sunlight absorbed. It's a funny thought I have sometimes: my younger self, in surface rebellious fashion, would take pride in not having a dreaded 9-5 just to be outside the mold.

Edit:
I just saw it said ONE habit. Fuck, I'm bad at this. Actually one of my major weaknesses: being indecisive and narrowing a "best X" to only one.