The most indispensable Festool is the one you don't have (when you are in a shop) or the one you forgot to bring with you (when you are at a site).
I thought that was obvious... [laughing]
EDIT: more seriously, the CT is really a big part of what makes most of the other tools as great as they are; this is clearly not the only thing (considering the drills as a case in point), but given how much value it adds to nearly every other tool in the lineup, would have to agree with the CT units if picking just one - not because of what they do on their own, but because of the value they add to the other tools.
I thought that was obvious... [laughing]
EDIT: more seriously, the CT is really a big part of what makes most of the other tools as great as they are; this is clearly not the only thing (considering the drills as a case in point), but given how much value it adds to nearly every other tool in the lineup, would have to agree with the CT units if picking just one - not because of what they do on their own, but because of the value they add to the other tools.