I've taken so many over the years (and that's even with doing a reduced number due to time constraints) and they've all been fantastic.
Off the top of my head I think Wand-making is probably my top pick because I got a beautiful wand out of it (let's not mention the beautifully manicured nails that were sacrificed in the making of, though). More than that- it was thought-provoking and relied a lot on personal insight. The lessons were also amazingly interesting.
For sheer fun factor and creativity though, Doctor Who class in my second year has to get a massive mention.
But, as ever, there are too many awesome classes out there and too little time!
"Does the Walker choose the path or the path the walker?" -Garth Nix
(Av/sig by Amy!)