The first thing you should really consider is to have your rabbit spayed/neutered. This will not only help prevent a near immediate conception once the rabbits are introduced (if they are of breeding age), but will usually prevent aggressive behavior due to territorial conflicts.
There is really no best answer to your question as all rabbits have their own personalities that either conflict with or compliment another rabbit's personality, much like with people.
Another thing to keep in mind, is that rabbits are territorial. Allowing a new rabbit into your rabbit's already existing territory could lead to fighting. Both male and female rabbits are territorial.
The best pairings for rabbits are mother/daughter, sister/sister, or brother/brother (only if neutered, otherwise they will constantly fight). Generally, females will get along better with other females as opposed to males being together. However, if all parties are altered (spayed/neutered) then they should all get along.
If introducing a young rabbit to an adult, I would suggest that the adult be a female, so she will feel less threatened and more "mother like" to a younger rabbit.
When you do introduceyour rabbits together, put themin a neutral area (such as a bathtub), so they don't have to feel like theyneed to defend their territory, and give them some enticing treats so they canquickly learn to share their food and boundarieswith each other.
http://www.inlerabbitry.webs.com