Bowline - the yachtsman favorite.
The Bowline is best for forming a loop or eye, it doesn't jam and it's easy to undo if not under load.
Some people find the bowline easier to learn by saying "the rabbit comes out of its hole, round the tree and back down the hole again".

Clove Hitch
Although it's not a very secure hitch, but it's quick to tie and easy to adjust - ideal for securing fenders and the like.

Reef Knot
Probably one of the most popular and best known knots.
Typical uses: tying the ends of a rope around an object, eg. a boom, a parcel, bandage, or the neck of a sack.