I am applying for UK Visitor Visa for 2 years for my business visits. I am registered as a One Person Company and therefore a bit unsure if I should fill the application form as Employed or Self-Employed?
The important thing for visa applications is to be clear.
They're not going to reject you for ticking the wrong box if it is clear to the ECO what your circumstances are, and that you're not trying to hide anything. No matter which box you tick, you will need to provide additional documentation of your income situation, which will give you ample opportunity to explain exactly what is going on.
On the other hand, they may nick you for deception if you make it look like you're employed by a corporation and it looks like you're hiding the fact that it consists of just yourself.
It does feel reasonable to let your tick depend on whether the company is a separate legal entity or not. But do not let that decision alone lead you to omit relevant information. For example, as the owner of a one-man corporation you'll want to document the company's situation and show its annual report; just documenting that the company (as a black box) pays you a monthly salary is unlikely to fly.