It depends entirely on which planets are involved. Working simply,by using the same pair of planets, the traditional order of strength would be:
Opposition
Square
Trine
Sextile
So on that basis the opposition would be seen as stronger, and you will notice that the 'hard' aspects are stronger than the 'soft' ones. Modern psychological approaches would explain this by reference to challenges, and drivers - stimuli to action, usually in circumstances that present obstacles that need to be confronted.
Traditional explanations revolve around oppositions having characteristics of Saturn (which opposes Sun and Moon in the sign rulerships) and Mars (which squares Sun and Moon in sign rulerships) - however whichever approach you pursue, oppositions and squares are the more difficult to deal with.
Once you stop using the same pair of planets, things become more complex. An opposition between Venus and the Moon is not as awkward as as square between Mars and Mercury. It is really the planets and their nature that matters, the aspect simply acts as a facilitator for their sharing - in the case of oppositions and squares that facility makes the sharing a little more difficult.
