707 for me, if only to complete the Boeing jet range (I assume I will get on a 787 flight at some point in the future). Not flown on a 717 either, but regard that as a McDonnell Douglas jet not a true Boeing!
Will always be frustrated that I never flew on Concorde as Mrs CP and I had been looking at doing just that when its demise was announced which instantly put an end to more accessible fares (we didn't come up in the ballots for tickets needless to say).