Congrats, Dev - I hope the system transfer goes incredibly smoothly, dude
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I'm pretty new to MMOs, having only started playing them in the past two years and, even then, only a little. I've played two games, both on PS4.
I started with
Final Fantasy XIV Online. Perhaps unusually, I hadn't played a Final Fantasy game before this one. Even so, I found the gameworld and characters to be quite charming. Technically the game is very good, allowing touch pad and keyboard use on the PS4. The game works is expansive, with the original A Realm Reborn world since supplemented by Heavensward and Stormblood. The game is subscription only but the accounting is setup outside of the PlayStation network and so you can access the PC-style pricing. I pay around £24 every three months. I'm quite happy with this as the game is well-maintained and regularly updated. It is extremely popular at the moment as Stormblood has only just launched! You can try out the game for free; up to character level 35, I believe. The game allows for cross-platform play (PS4 and PC, too).
My second game is
The Elder Scrolls Online. This game has changed a lot since launch, with new DLC areas added, keyboard chat supported, and the entire game world opened up so that's you can travel almost anywhere without having to rank up through the levels. Presentation is great and the tone is a little more serious than FF XIV. You can play without a subscription - and it's worth doing that to try out - but paying the monthly sub (£9.99 on PS4 through the Store) enables you to have access to all of the DLC areas (albeit not Morrowind, which is a separate expansion) and to have a craft bag (which is great as it's a limitless store for all of your crafting gear).
Being new to MMOs and unsure about... well, what to do, really... I found that there is a pretty steep learning curve in both games; there is so much to take in! I've really enjoyed both, though, and I plan to put more time into them rather than buying other games.