The best version of The Hound of the Baskervilles is the 1939 version with Basil Rathbone as Sherlock Holmes, but this version is very good. It's from Hammer Studios, and the plot was changed to make it more of a horror movie, but it works pretty well.
Peter Cushing is an excellent Holmes, one of the very best, and André Morell is tops as Watson. Christopher Lee is the romantic lead here, and he's okay. He towers over the rest of the cast, being a head taller than any of the others. I really enjoyed Miles Malleson as the comic relief bishop. The movie's in color, and it looks great for a low-budget production. Plus -- quicksand!
I don't want to spoil too much for you, but the hound in this one is real. Aside from that, I won't say any more about the plot. You should know it well by now, and while it's somewhat different here, it's close enough to the original to be both familiar and new. This version of the story is definitely worth a look.