All time great? Nah. Not even close to being part of that body for work, even for just progressive rock bands. They're more like a footnote - A really long and interesting footnote, but still a footnote.
The "problem" I have with grouping DT into the same echelon as Yes, or Rush, or Floyd, or some of the truly great progressive rock bands is that DT never took as many creative chances as those other artists. Everything they've done has some immediate and obvious precedence in the past, and nothing they've done has ever really felt that "new" or off-the-wall. So what did DT do? DT refined the progressive metal genre and were successful as its bannermen for an extended period of time. That's great, but it's not that important. Other bands, like Radiohead, did a lot more to innovate of the principals of progressive and experimental rock music, while being far more influential. Meanwhile, DT are about a safe and conservative as progressive rock gets, at least anymore.
Don't get me wrong, they're a personal favorite of mine, for different reasons - not because I think they belong in some all time great category.