Wow - those Kesslers and that Coiler are sweet.
Anyways, fun OP question. My 1 demo would be a SWOARD. My reasons: I would use this demo "opportunity" to try something totally different. I ride GS boards (see my profile). And, I generally have a racer style of riding; not EC really. And, I've always wanted to ride a SWOARD. In fact, at SES in 2008, I was going to hop on a SWOARD after I got off of Frank's metal Berserker. Well, thanks to Frank, I ended up on the Berserker all of the late afternoon that day; and then I took it home...
I must say Bullwings idea about the Tinkler is a great one. Very tempting. I'm always very envious when Brian posts shots of Tinkler's amazing boards or the two of them riding at Mt. Hood Meadows.
Wish I could say I'll be at SES to try one of those Kesslers. Interestingly, I see I'm a bit different that may of those who responded. If I had 1 and only 1 demo, I would not demo a Kessler. First, I ride my Kessler 75% of the time [so it would not - I don't think - be totally different to me] and, second, I know that I'll get another Kessler in the future.