questions about Fire mon

I ques Moltres! Double weak to rock doesn't bother, do not use it against rock. Better than flareon after all.

Entei much better against Ice types, Moltres better for grass and bug

In general Entei has higher hulk and Moltres kills faster. The bulk bit is compensated with Moltres flying typing (vs grass and bug) but also limiting it in the ice matchups.

Moltres is the only one I’ve taken past 30. The combination of its power plus resistances make it the best fire type currently as far as I’m concerned. Yes, Entei has more bulk, but a pure fire type currently isn’t really needed. Unless I’m missing something, there are no relevant pure ice types until Regice in gen 3. And even then, fighting would fill that role. As an aside: I really wish Entei was more useful. I caught a 98% one (my best legendary so far) on the last day and would love to power it up. But I can’t justify it.

While your question was which one is better, it's also advisable to look at candy cost. In my opinion, Moltres is the better specialist, and that is the best use for a fire mon. That's why I invested in kill speed and specialist role (Moltres) and Flareon as a pure fire type (more of a generalist and candy readily available).