I love Overeem - I honestly think he would be HW champ if his chin wasn't completely gone.
I was actually a fan of his brother Valentine back in Rings (when he submitted Randy Coutoure) and my first time seeing Allistair was when he fought Bobby Hoffman at a King of the Cage event. Seeing him go from a wiry kick boxer with promising talent, to a hulking super human that KO'd Badr Hari, Aerts, Spong etc. was probably one of the most startling transformations in all of mixed martial arts.
When Allistair won the Strikeforce belt, there was an aura surrounding him that he was damn near invincible - many people on the forums thought that he would be the first guy to defeat Fedor. Somewhere along the way his chin got cracked, and he downsized into Econoreem. Still an entertaining fighter, but a far cry from the beast that wrecked Brock Lesnar.