My personal opinion based on experience:
* GPT-5.1 has the best instruction following, strong agentic capabilities, and very good skills in math, coding, and problem solving.
* GPT-5.1-Codex-Max has worse general capabilities than GPT-5.1, but is noticeably better for large and complex coding tasks.
* Opus 4.5 has the best implicit intent understanding, very good instruction following and agentic capabilities, but lacks depth in its reasoning that is required for complex problem solving.
* Gemini 3 Pro has the best raw intelligence, especially in math, and has good agentic capabilities, but lacks instruction following.

So, the choice becomes quite simple:
* For well-defined general tasks, go with GPT-5.1.
* For well-defined coding tasks, go with GPT-5.1-Codex-Max.
* For less defined or ambiguous tasks, as well as general agentic scenarios, go with Opus 4.5.
* For math and problem solving in general, go with Gemini 3 Pro, but either pair it with one of the above or work on extra scaffolding.

I'm personally using GPT-5.1 and its Codex variant mostly, but sometimes trying Opus 4.5 and Gemini 3 Pro when the task fits them. I got used to the way you have to use GPT-5.1 and it gives very good results in any task I throw at it, if it's defined properly. For vibe-coding and front-end, I'd go with Opus 4.5 for its intent understanding in more ambiguous cases.

All these models are roughly in the same pricing league, but OpenAI and Google also have stronger beasts: GPT-5.1 Pro and (upcoming) Gemini 3 Deep Think. These are only available in expensive subscriptions for $200/$250 a month, and are very slow. But I'm still using GPT-5.1 Pro almost daily for better results in tasks requiring reasoning.

A very common scenario in my work is to throw all the context about the task into GPT-5.1 Pro and ask it to write a detailed implementation plan, then give that plan to GPT-5.1-Codex-Max to implement. It works out very well, especially if you do a couple of follow-ups with GPT-5.1 Pro to refine the plan and sync it with your intent better.

Gemini 3 Deep Think is a similar thing, and it will probably be even better for complex reasoning tasks, but due to the lack of instruction following in Gemini models and the fact that it's behind another $250 paywall, I'll stick with ChatGPT Pro for now.