Black Bolt with Space Gem
VS
Charles Xavier with Mind Gem
VS
Doctor Strange with Soul Gem
VS
Iron Man with Reality Gem
VS
Namor with Time Gem
VS
Reed Richards with Power Gem
More likely than not, Xavier would win as he's the one who could probably best use the Mind Gem.
Iron Man doesn't have the mindset to properly use the reality gem. It takes someone who can already manipulate reality to a minor degree to truly grasp it's use; heck even an Elder of the Universe couldn't properly use it. Thanos already had a degree of matter manipulation which allowed him to better use it.
Same goes for Namor; he doesn't have much experience manipulating time so it would be difficult for him to use it properly. Xavier on the other hand has had years to refine his mental powers, which would be exponentially increased when tapping into the Mind Gem.