Can you tell your best friend that you love him or her?

If this is a "true" friend, not merely an acquaintance, it is entirely acceptable to tell that person you love them. Bear in mind however that many people mistake "familiarity" for "love." The fact that you may trust someone and feel comfortable when they are around does not necessarily mean you "love" them.

There is also the natural physical attraction of men to women and vice versa that should not be mistaken for "love." If you think you "love" this person because you are sexually attracted to them you are way off base.

In any event, talk it out with your friend and see where it goes. That's what "true" friends do. What is the worst thing that could happen? You might have to find a new friend. My romantic side argues though that millions of beautiful relationships have been lost because friends were afraid to tell each other how they felt.

Yes you can. I believe male and female relationship does not exsist without some sexual tension.

I do feel that, when something rises slowly it will also go down slowly. Thus enjoying a wonderful, lasting relationship.

Sure you can

do you mean "I love you as a friend" or just "I love you" ?

the first one could go over great. the second one, could put some strain on the friendship if she doesn't feel the same way you do about her. (or swap the genders around for the other way)