Top 3 most useful:

• Telekinesis. At extremely high levels it also allows for pyro-, cryo-, and electrokinesis as you become able to effect atoms.
• Telepathy. You can read thoughts and emotions, communicate silently, trick people, or even puppet then around or brainwash them.
• Teleportation. You can safely go anywhere instantly.
• Also: matter to energy or other reality warping stuff. OP as hell and hopefully won’t cause a nuclear blast or turn the sky into lead.

Worst powers:

• Summon (something useless or random). You can summon useless objects (like a broken wristwatch) or just random / inappropriate things (a can opener when what you really need is $50.
• Speak with snails (or something equally useless).
• Realistic Photosynthesis. You are green and even if you’re naked in full sunlight you lack the surface area to survive on it. Also you have diabetes because your body is not designed to handle all the sugars you’re producing.

Drawbacks: honestly crack open the rules for GURPS. Here’s some honorable mentions.

• Unscalable. The power has only one one power setting, on or off.
• Always on. You can never stop using it.
• Uncontrollable. You have no control over when it activates or how.
• Shy. You can only use it when nobody’s looking.
• Growing insanity: The more you use it, the worse your grip on reality becomes.
• Restricted. You can only use it in specific circumstances, ranging on a scale of from “only at night” to “only from 5 feet away” to “only when mortally injured.”