'Monster' because of its incredible rhythms and vocal melodies, the subtle implementation of Orianthi's guitar play, and 50 Cent's great rap verse which, in my opinion, takes the song to another level. Unlike the spoken bridge on 'Hollywood Tonight' -- I like HT a lot too and I understand why the bridge is there, but I just don't think it enhances the song like the rap verse on 'Monster' does.
And last (but by no means least), because of the lyrics; I absolutely love the concept of Michael turning the mirror on Hollywood's "easy fame" lifestyle, and those who pursue that lifestyle thinking they can emerge from it unscathed, forcing them too take a good hard look at themselves and what they're doing -- he's basically saying "You wanted fame? Well now you got it. And now your scared, out of control and losing your mind, and you want out. Too bad. Deal with it." I think its lyrics are probably the strongest on the album.
'Behind The Mask' is a close second when it comes to the album's uptempo songs by the way. As for the slower songs... it's hard to pick just one, so I'll go with '(I Like) The Way You Love Me' and 'Best Of Joy'; great melodies, great vocals and great production on both of them.