[Bit_of_Ivory]

What spawn eggs and mobs are considered to be more valuable or useful on the server when you take into account trade, production, currency, rarity, etc? For example, obviously a wolf would be more desired than a bat because of the security, attachment, and value in trade, but what are some other valuable mobs/eggs that one should want to collect for whatever purpose?

(Also sorry if this question seems a bit obvious. I'm an older Minecraft player and even though I've been playing more regularly, there are still a lot of features, mobs, etc, that I'm not familiar with.)