Wow. What a question.
I'd have to say that my idea of great lectures has changed greatly over the course of my life, along with where I was in magic and my skill level and interests at the time. I honestly can't give an overall "best" -- just "bests" for their category...
I saw Darwin Ortiz in 1978 or '79 at Barry's Magic Shop in Wheaton and the moments _after_ the lecture, where he just did stuff without tipping it -- it made my brain whirl for the better part of a decade. A few years ago he was stunned to see those lecture notes when I presented them to him and he chuckled, "Oh, I'm _much_ better now."
For a flat-out hilarious lecture combined with real-world tricks and real-world bar-restaurant experience, Simon Lovell. Simon first convinced me of how you really *sell* an effect. plus, well, he's just _nuts_.
For sheer poetry, Rene Lavand. Period. Darwin did a great service in translating, but for once, the years of Spanish I forgot from my childhood in South America came back that night and I was able to see and hear what he was saying almost without 'losing anything in the translation.'
For learning how to 'act' as a magician, Whit Haydn. His ideas on movement and its relationship to script were wonderful. And he's great to chat about politics with at the bar at the end of the night at the Castle!
For diabolical thinking: Howie Schwarzman. He doesn't lecture much but he knows things a lot of people don't, and all you have to do is ask him.
For magical epiphanies on the issue of specific sleights: David Williamson on the top change, Tom Mullica on palming and Whit Haydn on "miscalling the double face down."
For contemplative thinking on the structure and meaning of any trick you do -- it's a tie, and they'd both laugh at the fact that I put them both here: Jamy Swiss and Eugene Burger. I've seen both lecture three times, and had both teach me things I probably would never have gotten without the constant mental beatings administered in genteel situations. Both have fried me, over and over and over again, both have contributed immensely to my personal growth in magic, and I'm proud to call both of them my friend.
Oh, and although it had absolutely nothing to do with anything I do from day to day, Danny Sylvester on creativity at the '99 Cape Cod Conclave. Just an amzing glimpse into the world of a guy who created an act totally different from the world at large, and how he went about doing it.
On the way to these few, I've seen a world of crappy lectures, but you know what they say -- discretion is the better part of valor...