There is no single answer to this question. It depends a lot on what you need to do with your domain. For example, for sure the most registered domains in the world are .com, .org, .info and .net. Which are also called generic extensions. These are good for generic domains, and if possible you should always register one of their extensions, even if you want to activate your site on an extension with a narrower niche. Otherwise you can register geographical extensions, if the market of your domain is restricted to a particular state or continent (for example .it for Italian domains, or .eu for domains within the European community).

Of course. The domain is always your property. You can decide to transfer the domain to us whenever you want. Typically almost all domains require a transfer authcode that you can request from your current registrar. You can provide us with this authcode, when you want to move the domain to us.

Except for sporadic cases, the registration of domains is regulated on a first-come, first-served basis. That is, the first to arrive registers. In this way, if you have obtained a certain popularity with a particular extension (for example .eu) it is possible that a competitor of yours registers a different extension to prevent you from registering it yourself. To avoid this the easiest thing to do (which does not involve legal name protection from an active tradermark) is to register the main extensions you think you want to use. Then configure them to redirect to your main domain.