Question

What radicals are most stable?

Answer 1

Radicals are most stabilized by two things:

1. Hyperconjugation, or

2. Resonance

The former delocalizes electron density from adjacent `C-H` bonds, and the latter delocalizes the radical electron around a molecule.

I'll refer you to another one of my answers to explain the first concept, because students find it hard to understand, generally.

https://socratic.org/questions/how-does-the-stability-of-carbocation-is-related-with-the-stability-of-free-radi