First things first: snakes are not poisonous, they are venomous. Poison is a toxin that you ingest or adsorb into the body (like if you swallow rat poison, or absorb frog poison through your skin), whereas venom can only harm you if it is injected into your body and enters your circulatory system (such as when an animal bites or stings you).
Secondly: snake venom potency is widely argued, and every list of "most dangerous snakes" you see will be different from the next. A snake's venom is highly variable, and can be more potent depending on the snake's temperament, age, or even time of day (a surprised snake that has just eaten a big meal is less likely to dish out a potent bite than an angry, hungry snake who has just been stepped on). Furthermore, the "most venomous" snake is not necessarily the snake with the highest death toll. Spectacled cobras are among the most "deadly" of snakes simply because they are responsible for a large percentage of deaths per yer in Asia. However, the spectacled cobra is not considered to have venom of absurd potency in comparison with other venomoids, and in relation to the number of bites they dish out, deaths are relatively low (6.5 - 20%).
Having said all that... It is typically regarded that Inland taipains, black mambas, certain species of krait, and Eastern brown snakes all possess particularly potent venom based on how quickly a subject may succumb to the venom. I recommend reading the Wikipedia article about venomous snakes. They have their own list of "most venomous," but like I said, it will be different from other lists.