If you want more monsters, with a variety of options for many of the 'official' ones, you can't go wrong with MME 1 to 3, and Dragonix Deadly Denizens (same author). I definitely like all four of them and they make the other third-party monster books mostly obsolete for me.
Bloodied & Bruised gives some additional actions when a monster first gets bloodied (down to <= 50% health) which mixes up the fight a bit.
In a similar vein there are the Recharge series which adds rechargeable skills and Legendary Bestiary series which adds legendary actions.
I like them, but having some actions in a separate book is a bit of a pain, i.e. you basically have to create your own statblock that includes these, to not forget about them.
Don't have the Legendary actions or Lair actions ones