* περίεργος (periergos): This translates literally to "curious" but is often used to describe someone who is odd, eccentric, or peculiar. It's a relatively mild and neutral term.
* παράξενος (paraxenos): This means "strange" or "unusual" and is a good general option for "weirdo." It's also fairly neutral.
* τρελός (trelos): This means "crazy" or "mad." This is a much stronger word and should only be used if you want to imply the person is mentally unstable, not just eccentric.
* τρελαμένος (trelamenos): Similar to "trelos," but implies a more temporary state of craziness.
* αξύριστος (axyristos): This means "unshaven," but can be used ironically or playfully to describe a dishevelled or generally unkempt weirdo.
The best choice depends heavily on context. For a general, relatively harmless "weirdo," περίεργος (periergos) or παράξενος (paraxenos) are probably your safest bets.