This is a tough question. Danny had the best overall season because of the championships, and he played on the best team (I'm comparing 1996 to 2007. Comparing 96 to 2008 is another discussion). That said, if you put Trask on that 96 team, without a doubt he wins the championships and Heisman with similar or maybe better stats. Thus, Wuerffel was not the "most impressive" QB of the 3 imo.
That leaves Tebow and Trask. Trask broke conference and school records on a lesser team, with a worse O-line, and a scheme that isn't the perfect fit for his skillset. Tebow also broke school and conference records, but unlike Trask he also broke national records becoming the first player to have 20/20+ pass/rush TDs but he also did 30/20+ shattering the record. Tebow had the better team, but also had the tougher schedule. Trask impressively will finish with more total yards, but Tebow will finish with more total TDs (I know Trask has 2 fewer games but it's doubtful he would have as many TDs if he played 13 games).
While I think Trask is easily a better passing QB compared to Tebow and has a much better chance of NFL success, I think Tebow had the most impressive season. And the reason why is because he did things that no one had ever seen a QB do before. Trask was spectacular, but he wasn't extraordinary.