I've usually done it with a hammer - if a shed has got to the "it might as well just go" stage, then it will normally knock apart quite easily. A saw can be useful, if the floor is all in one piece, or if OSB has been used for repairs, for example. A grinder and/or a jemmy can be useful for separating the door hinges, if that become necessary.
Get any glass out first, of course - and beware of it falling on you, if you remove a critical part too early on.
If the sheds are "OK" and just no longer wanted, then you could find that somebody might want them and take them away for you. They are usually dismantlable, with thought and care.