What a Witcher 3 trophy is worth
Trophies drop from contract monsters in The Witcher 3 and do nothing in the inventory. Strapped to Roach they give a passive bonus, usually extra experience, better prices from merchants or a resistance, and only one can be equipped at a time.
Each Witcher 3 trophy here lists the exact bonus from the game files and the contract that produces it, so the choice comes down to what a playthrough needs rather than which monster was hardest.
Getting monster trophies in The Witcher 3
Trophies come from witcher contracts rather than ordinary kills, which makes the Witcher 3 quests page the practical list to work through. The monster itself is in the bestiary with the oil it takes.
A trophy shares the horse with saddlebags and blinders rather than competing with them, so there is no reason to ride without one once the first contract is done.