I'm in the Middle of Act 2 and have several open slots for mutagens and several Greater Critical Effects, Greater Vitality and Greater Power mutagens. The only regular one I don't have is Greater Enchantment, and I just unlocked my ultimate, in which it would help a little.
Judging by the prices, Greater Vitality is the best I have.
Question: Will there be amazing rare mutagens in the future, or this is all there is?

I'm personally a fan of using all Greater versions of whichever mutagen it was that adds +% to knockdown, incernate, etc.

There are a few mutagens, as far as I can tell, that aren't of the typical lesser, norm, greater, but I never used any of them (increase sign range etc). There's no special uber super secret unique mutagens either, if you're holding out (as far as I know).

And, just to clear confusion, mutagens don't affect the skill they are tied to. They are global, in that they affect you, the character. So don't worry about using a mutagen you think would affect that specific skill.

I'm quite aware of that. The reason I'm holding out is if I use all my slots (I liked the Critical Effects Mutagen too), and then some vitality regeneration during combat mutagen comes up, It would be a shame.

Guys use the mutagens don't worry i did hold and hold and at the end finished the game without using mutagens lol