I went for Cataclysm. I might be regretting it now. I love them all, they are well directed and watching how they're made is fun (on the discs on the Collector's Editions). Maybe I'd go for the WotLK one instead, even though I don't know all of Arthas' backstory.
On a sidenote, the best moment in the TBC cinematic is the draenei male wielding his mace and swinging it back on the enemy behind him.