If I'm playing straight D&D, the ability score tables I use are (closest to, but a little more complex than) the ones from
Swords & Wizardry:
3–8, −1
9–12, ±0
13–18, +1
If you're rolling 3d6 down the line, each stat has about a 25% chance of a −1 penalty, a 50% chance of no modifier, and a 25% chance of a +1 bonus.
In lots of D&D-derived games set in other genres (like steampunk, modern, or sci-fi), I'll use a table which is essentially the "d20 System" table cut in half:
3, −2
4–7, −1
8–13, ±0
14–17, +1
18, +2
This table leaves characters with a two-thirds chance of no modifier per stat, and only a one-sixth chance of having a penalty or a bonus. I used it in writing
Engines & Empires, and I'm given to understand that Kevin Crawford did the same for
Stars/Worlds Without Number.
(The S&W-derivative
White Box: Fantastic Medieval Adventure Game uses an even sparser table: −1 for 3–6, ±0 for 7–14, +1 for 15–18. That makes bonuses and penalties only 10% likely each, and 80% of rolls will give no modifier. That's too rare even for my tastes! But I can see the appeal of it.)
I don't use anything like exceptional Strength or extra hp for fighters with high Constitution. (Even when I was more inclined to play AD&D by-the-book, while I did keep the high hp bonus for fighters — and also granted it to monks — I made sure to
completely eliminate exceptional Strength from the game and replace it with another mechanic.)