Anything from Andrew Lloyd Webber, I'm a self confessed 'The Lord' fanboy. Haven't seen most of his stuff live, sadly, but the music? Epic win. And I saw the touring version of Joseph a couple of years ago, and I so desperately want to see Cats when it's next performed in the UK (since sadly the West End version ended a few years ago after a revival in 2004), and Memory is the best piece of music from a musical I've ever heard. Susan Boyle definitely did it justice
Haven't seen anything from Stephen Sondheim, sadly, but I'd love to see Sweeney Todd, the film was great (and I was pleasantly surprised to see Johnny Depp, Helena Bonham Carter and Alan Rickman all have amazing singing voices). In general, I'm a musicals fanboy, though I can't sing myself :s
Les Miserables is pretty damn high on my list too, as is the Beggar's Opera (missed my chance to see it back at my old school when it was performed...). I'd also love to see the Lion King, and I saw We Will Rock You in the West End a few years ago. That show was bloody amazing and my brother was in a performance at his school a few weeks ago, sadly I didn't get to see it since I'm at university, but it was apparently really good, he was a minor role as 'Jackson 5' of the Bohemians. His best friend, one of my friends too, apparently stole the show as Pop, but then again the guy is a natural born rocker...