Codex: Three reasons. I've used all extensively, for multiple months.

Main one is that it's ~3 times slower. This is the real dealbreaker, not quality. I can guarantee that if tomorrow we woke up and gpt-5.2-codex became the same speed as 4.5-opus without a change in quality, a huge number of people - not HNers but everyone price sensitive - would switch to Codex because it's so much cheaper per usage.

The second one is that it's a little worse at using tools, though 5.2-codex is pretty good at it.

The third is that its knowledge cutoff is further in the past than both Opus 4.5 and Gemini 3 that it's noticeable and annoying when you're working with more recent libraries. This is irrelevant if you're not using those.

For Gemini 3 Pro, it's the same first two reasons as Codex, though the tool calling gap is even much bigger.

Mistral is of course so far removed in quality that it's apples to oranges.