The apparent impasse when two or more protagonists can each kill the other in the event of an attack.
The 'Mexican Standoff' is a favourite dramatic device, as it gives ample time for dialogue, facial expressions and uncertainty over how the situation will be resolved.
The reasonable solution in such a situation would be to negotiate a tactical withdrawal (as in Tarantino's "Kill Bill", when the Bride discovers she is pregnant and convinces her would-be-assassin to leave), but of course, people are rarely reasonable in the movies.
It is interesting to note that such a situation can only arise when the mutual destruction of everyone involved is fairly certain. This generally requires a certain level of technology, in order for the weapons involved to be sufficiently foolproof and deadly. For example, crossbows would probably qualify, but swords would require each person to have a blade at their opponent's neck (which has happened in some films). Explosives rigged to hair-triggers have also been used to successfully engineer a standoff.
Rare exceptions aside though, the overwhelmingly traditional weapon of choice for this category will be the firearm.