There are 3 broad strategies here:
- gardens deck
- buying colonies with treasures and margraves, probably in some sort of engine with plaza
- possession in a margrave/plaza engine
The way that you play these broad strategies will change depending upon your opponent's strategy too. A rough evaluation will tell you that gardens will beat possession, possession will beat treasure/colonies, but it's not easy to evaluate gardens vs treasure/colonies. It needs some maths. The best gardens deck is going to use beggars and it will collect a lot of copper, it may collect a lot of silver if the opponent uses margraves, so it can score 8x5 for gardens maybe, plus a lot of duchies, meaning that an opponent would have to buy most of the colonies to beat it. If beggar/gardens looks like it beats treasure/colonies by this match up then it's just the best strategy so use it.
Let's suppose however that treasure/colonies will be better than gardens. We then have a gardens > possession > treasure+colonies > gardens dilemma. The solution comes from the flexibility to switch from a possession plan to a colony+treasure plan if the opponent starts buying beggars for gardens. A gardens player cannot switch strategy once they have started, even if they get hopeful with beggars and banks.