For LLMs right now, I think of it as four ‘speed tiers’:

1. Quick and easy. You use this for trivial easy questions and ‘just chatting.’
Matter of taste, GPT-5 is good here, Sonnet is good here, Gemini Flash, etc.
Most of the time you are wrong to be here and should be at #2 or #3 instead.

2. Brief thought. Not instant, not minutes.
Use primarily Claude Opus 4.1.

3. Moderate thought. You can wait a few minutes.
Use primarily GPT-5-Thinking and back it up with Claude Opus 4.1.

4. Extensive thought. You can wait for a while.
Use GPT-5-Pro and back it up with Opus in Research mode.
Consider also firing up Gemini Deep Research or Deep Thinking, etc, and anything else you have handy cause why not. Compare and contrast.