The strongest mutant are among the likes of Absalom Mercator or Franklin Richards for example. Reality warpers. They can pretty much do anything but thankfully, such powerhouses are either not interested in domination or are psychologically challenged and can't control their powers. A good example is David Haller (Son of Charles Xavier), it's hard to know his limit but he is crazy as a bat. So, in the end, he has multiple personalities and each personality have a distinct power. Scarlet Witch is an other example (twin sister of Quicksilver, daughter of Magneto). She is extremely fragile but under some circonstances, she is an absolute monster. One day, she had enough and she said 3 words "No ... more ... mutants". Conclusion: only 198 mutants left in the whole world. She depowered thousands, millions of mutants with 3 words. And this wasn't temporary. It was used to reboot the X-men comics.
What makes Apocalypse so dangerous is the fact that he is ... I was going to say sane but "fully functional" is maybe a better word.
Even if he is not a reality warper, his powers are still over the top though:
- As strong as the hulk
- Telekinesis almost stronger than anyone
- Immortality
- genius
- Surnatural gift towards technology (and this was the source of multiple other powers)
- ...
You can read more about him here:
http://marvel.wikia.com/En_Sabah_Nur_(Earth-616)
I highly recommend this webside if you want to know anything about the marvel universe