There are a lot of debates on who's the most powerful god: Zeus, Poseidon or Hades? This blog is about my verdict on them. Feel free to comment.
First of all, there are two types of Power: the Influential type and the Supernatural type. So the most powerful god according to you depends on your definition of power.
Zeus is the most powerful god in terms of influence. After all he's the King of the Gods and was honored in all Greek Cities. Also because most Olympians and gods are either his children or his sisters.
Poseidon comes next, because he ruled over the oceans (which the Greeks depended on) and was the Protector of Greece. Hence the Greeks both feared and respected him at the same time.
Hades is the least influential god because he was the god of the dead and he was feared more and respected less. Still he wasn't an evil god as modern portrayals depict him. He had evil characteristics but was a good person at heart.
Supernaturally though, all brothers are equally powerful. Zeus has powers over the air, weather, the effects of Time and electricity. His lightning bolts were charged with a lot of raw power, so that makes Zeus very powerful. Plus, Zeus could control the Sun, Moon, Stars, Ageing and the Life Force. Hades has amazing powers over death, darkness, metals and dreams (he controlled the Oneiri, dream spirits) and his helmet could radiate enough fear to drive a lesser being insane. Also his absolute power over the dead was very handy.
In terms of versatality, Poseidon is way ahead of his brothers, though he's of the same overall power. Afterall, he's the god of the Oceans, Natural Disasters, Equastrians, Marine Life and Marine Weather, Winds, Ice, Storms and occasionaly, he could even control Lightning Bolts too (though not upto the same extent as Zeus). Plus he was said to cause mental illnesses from time to time.
So basically, Zeus has raw power, Hades has very handy abilities and Poseidon is super versatile. All of them are equally powerful, so take your pick.