Determining the "best" MMORPG on Steam is subjective and depends on individual preferences. As of the time of this response, some popular MMORPGs available on Steam include:
1. Black Desert Online: Known for its sandbox gameplay, deep character customization, and intense action-based combat.
2. Star Wars: The Old Republic: A story-driven MMORPG set in the Star Wars universe, featuring diverse gameplay and iconic locations.
3. Guild Wars 2: A popular MMORPG with a vast, open-world environment, a strong focus on storylines, and a dynamic event system.
4. Final Fantasy XIV: A critically acclaimed MMORPG with an engaging narrative, deep job system, and an active player base.
5. Elder Scrolls Online: Set in the Elder Scrolls universe, it offers a vast, open-world experience and multiple progression paths.
Remember that the best MMORPG for you depends on your personal preferences and interests. Many of these games offer free trials or limited access, so it's a good idea to try a few before deciding which one meets your needs best.