What I have to say at the beginning - boss-hunting nowadays isn't profitable at all, so don't expect too much. Why? Because to achieve good results you have to spend too much time on it compared to the profit you are able to get.
If you think about expensive rare items - then the most profitable can be Ocyakao with an Eye of the Storm (sth like 50-70 kk on Antica)*, Tyrn with Sun Mirror (about 20-40 kk on Antica)* and Furyosa with a Furious Frock (30-50kk on Antica).*
But to be honest - loot one of these item is something like "mission impossible", you need huge luck and the competition of other, experienced bosshunters may be irritating.
If you really want to hunt bosses only to have some money - then I suggest to focus on Zushuka and The Welter (...and depends on how wanted are items like Vampire Lord Tokens and Glass of Goo on your server - vampire lords and Shlorg), because drop chance to loot rares from these bosses is much bigger and it's quite easy to sell the items.
For example now I've got two Glass of Goo for sell... for 3 months nobody wants to buy it, is it really profitalbe then?
I have to mention about Zulazza The Corruptor (Dragon Scale Boots are very useful and expensive nowadays) and The Pale Count which is still worth to kill.
On the other hand, since Cipsoft has implemented Midnight Panther, Undead Cavebear and Yeti to the bestiary, you can treat it as "profitable boss" and ask 5kk for finding it for other players, hah
If I were you, I would skip rest of the bosses, and focus on daily bosses like Dream Court - arena and The Nightmare Beast, Grand Master Oberon, Scarlet Etzel and finally King Zelos. It's a 100% success and with a little bit of luck your profit may be really impressive.
* note that pizes may change over time