Hmm, interesting question. Certainly, all of them were influential. The question is how many of them were so unique as to be irreplaceable. I would tend to think that the answer should be the death which would have led to the most immediate change, since if a founder's death only causes a change latter on (i.e. in the case of Madison), butterflies would obscure matters greatly.
In this light, I think there are two contenders: George Washington and Thomas Paine. John Adams might be another, but his marshalling of the movement for independence was a group effort; and of course, Adams himself was sometimes the movement's biggest liability. As to Washington, it's hard to tell. There were other military commnaders that might have filled a role if needed; there's also something of a cult around the figure of Washington that makes it hard to assess his indispensability. Nevertheless, I've read enough TLs with a successfull (enough) revolution that I think you could have found a different commander-in-chief (you will however produce a very different USA).
I tend to think the choice might be Thomas Paine, assuming that his death also precludes the composition of Common Sense. The pamphlet had a huge effect in fueling the outbreak of truly republican sentiment in the colonies on a large scale. Now, removing the pamphlet doesn't drastically increase the number of loyalists; however, it might well mean that the number of "those in the middle" is increased. This in turn means the Revolution takes on more of the over tones of internal civil strife, with Patriot/Radical against Loyalist/Tory to a greater extent than OTL. I'd argue this a greater change because it will fundamentally alter the nature of the American Revolution. Just my two cents, though.
