Welcome to Gaia! ::

D class:

Destructive Capacity: Wall level to Wall level+

Speed: Superhuman

Durability: Superhuman

Strength: Superhuman

C class:

Destructive Capacity: Small building level to Small building level+

Speed: Faster than the eye can see, likely supersonic to low hypersonic

Durability: Small building level to Small building level+

Strength: Superhuman

B class:

Destructive Capacity: Large building level to City Block level

Speed: Low single digit Hypersonic+(Mach 7+) to double digit Hypersonic+ (Mach 16+)

Durability: Building level+ to City Block level+ (likely higher)

Strength: 20+ tons to 1000s of tons+

A class:

Destructive Capacity: Multi City Block level to Hill Level+

Speed: Low double digit Hypersonic+ to mid double digit Hypersonic+ (mach 22+)

Durability: Multi City Block level to Hill level+

Strength: Class 100

S class:

Destructive Capacity: Mountain level+ to Island level

Speed: Mid double digit Hypersonic+ to high double digit Hypersonic+ (probably ultra low triple digit Hypersonic)

Durability: Mountain level to Island level

Strength: Class 100

Note: The speed of S class demons is still being debated by the fandom.

Did the manga ever state what level was the lowest? I feel like there should be a level [or more] below D-class for normal humans and super-weak demons. Since the Reikai uses the system to classify demons, I'm pretty sure they would never use it for humans, but it'd be good to have a benchmark, so to speak.

IIRC there is no class below that of D class. I don't think Reikai would ever bother to classify normal humans. Sensui is low S class, Kuwabara is A class and Genkai was high/mid B class before giving her powers away to Yusuke.
I can always made a fan made tier though: smile
E class:

Destructive Capacity: Low street level
Speed: 1 m/s to 10 m/s
Durability: Human level to peak human level
Strength: Base human to peak human

For reference, take a look at these:
Destructive capacity
Durability
Speed
Strength
Peak Human
Power Rankings