Actually, potassium reacts the most vigorously.
Lithium dissolves gently and calmly, almost boringly.
Sodium reacts vigourously, skittering around the surface of the water until it melts, then it tends to go bang (if it hasn't already been used up at this point).
Potassium goes bang almost immediately. (Usually when I do this, there's lovely purple flame for about 15 seconds before there's a huge bang.)
This naturally depends on the amount of alkali metal used, and how coated it is in oxide.
For an explanation, recall that they're each losing an electron. As you down the colunm, that electron gets farther and farther away from the nucleus, and so it takes less energy to be removed. Thus, of the energy given off by the reaction of water with the additional electrons (giving hydrogen and hydroxide), there's more left over to go into heating up the metal and the water.
The hotter the water and the metal get, the faster they react, especially once the metal melts (liquids always react faster than solids, all other things being equal).
The melting point of the alkali metals also decrease going down the column, so potassium melts faster than sodium, and lithium never melts in these reactions at all.
SO lithium would react more?
