The conjunction is the strongest aspect. Its influence strongly depends on the energies of the planets involved.
Put simply, a conjunction represents a close bond and interpenetration of both energies. The planets involved continually influence each other. If these planetary energies are similar - for example Moon/Venus or Sun/Mars - the conjunction's potential is increased. In this case, the planets support each other and in combination often find it easier to assert themselves when facing others. If two very different planets are close together - for example Mercury/Saturn or Mars/Neptune - the potential in each may end up being inhibited.
Which planet is dominant depends on the sign and house position of the conjunction together with aspects to other horoscope factors. A Mars-Neptune conjunction in a water sign could mean that the owner of the horoscope finds it difficult to be decisive or self-assertive. On the other hand, this very quality can help him/her to use subtle means that others may underestimate to achieve his/her goals. The same conjunction in a fire sign could mean that a person's intuitive traits are not so obvious owing to the dynamic energy inherent in such a Mars placement.
The astrological term for several (i.e. more than two) planets in conjunction is a stellium.