TBH, I don't spend that much time with the butterflies (Not really into insects). I tend to head straight for the otters. It is important to go at feeding times as you are guaranteed to see the otters and the keeper will give a talk as he/she feeds them, raw meat but nothing alive.
It can get a bit cutthroat when it comes to getting a good position when moving from pen to pen so a longish lens is always a good idea.