1. Diamond Golem of Maino Shaman from IGG Castle.
It has tons of HP (only 20000!) but the drops really worth it o.o This is my favorite Boss so far, you'll understand why ;)
2. Maino Shaman (2nd Boss of IGG Castle)
It dies faster than DG (Diamond Golem) but however, it's impossible to solo with it's very high HP reggain. It's rewards are good as well!
The Rough Diamond and the Tail Hair of Maino Shaman worth 100k each. Both bosses can drop n/b zels and dais :)
3. Barbados from Heine Castle
This one hits really hard, a mage is necessary to defeat it. The drops aren't so bad but they could be better, the high end rewards are very rare (Oriharukon Dagger, Holy Walk spellbook and of course Poly spellbook).
It dropped few Eva's Scorns but they're hard to sell considering they are crappy weapons. Anyway, we find it up often and it dies quick!
4. Spirit from Polluted Land of Blessing
Spirit would be much easier if it would not spawn any servants, they are annoying x_x I usually waste all my mana to kill them q_q but I can't complain about the drops, every time we kill it, we obtain at least one blessed scroll.
Another interesting fact about Spirit is that all it's parts (Leaf, Bark, Root etc) worth 30k each :) I haven't gotten the Spirit Defender yet but it seems to be hard to sell anyway :P
5. Aaarrrggg... from Kent Castle
The last one is a weird named boss called Aaarrrggg... o.o Regardless of it's sucky name, it looks pretty cool. It won't be aggresive on sight and I would suggest to kill it's 4 Vampire servants first. It seems to spawn less often than the other bosses but it never been a waste of time to kill it. The lines are great :)
Since the update, it always dropped 1 B-dai and 1 B-zel and the Kent items can be sold for Adena :D