IMO opeth didn't come into their own until Still Life.. There are some good moments on orchid and my arms your hearse, but nothing compares to how their sound developed on Still Life..
Also if you don't have black water park shame on you! That is my personal favorite.
But really anything from still life moving forward is top notch.
I'm not really familiar with sonata arctica.. I will have to give them a 2nd listen.