I need to send a image as the title says. But the amount of RGB choices confuse me... There's sRGB built-in, sRGB, adobe RGB, apple RGB... I thought of choosing "Best RGB", is it okay...?
sRGB is almost always the correct answer.
If you have not been specifically told by the printer you're sending the photo to, which profile to use, then use sRGB - as anything else is quite likely to generate incorrect results.
sRGB is the profile used by computer desktops, by the web, and by many print shops too (you may be surprised). A modified form of it is used by television and video. It's the default used by digital cameras, too. A few print shops may support Adobe RGB or even other spaces, but if they do, they'll make it clear and they will know what they are talking about, because colour management needs specific knowledge. It also becomes difficult to preview and edit a file as soon as you use a colour profile other than sRGB because even in the event you have a colour-managed editor and you've configured it correctly, it will still probably distort it down to sRGB in order to display correctly on your graphics card and monitor.
sRGB is a relatively narrow-gamut colour space, meaning that highly saturated colours may be "clipped" or truncated, particularly when it comes to brilliant blue-green colours such as those found in peacock's tails, butterflies and iridescent objects. Thus there is a very high temptation to use a wider colour space such as Adobe RGB or something else. Unfortunately, correctly previewing and editing in such a colour space is just that much more difficult.
It depends on the destination of your image.
For web, sRGB is the standard. Unfortunately, not all browsers acknowledge your embedded profile so you have to destructively convert the colors.
For working with your photography, Adobe RGB will retain a much broader spectrum.
Here are a few questions on GD.SE that may help explain the profiles.