I would say probably Draco, although it's mainly the A3 that you want booked, so you haven't been lucky so far. And it does depend to some extent on your epics - if you have a booked Tayrel or Stag Knight, maybe Draco's A3 is a bit less important than it would otherwise be - though it's still better than what they have. 

Thanks

I booked him and I got the 2 reduced cooldown on the A3.

However I find it sometimes redundent when I use Serris who also has decrease def but also apply a decrease attack in addition.

Reduced cooldown on Maulie's revive would be nice as well but I have been told to focus on Big'Un after ...

I personally have both maulie and Dracomorph. I very rarely use maulie and use draco in all but fire knigth. So my recommendation is wait with books on maulie until you have enough legendary tomes to book whoever you pull next.