The old reliable:
Spiral Knights
It's a bare bone top down dungeon crawler, go as far as you can in a single run, no microtransactions, designed around a part of 4 but you can solo it, extremely cute customisation. The game has been on life support for 14 years, always active despite the small community, the thing literally won't die.
It's a pick up and play game.
I've been playing since 2011, originally got it for the TF2 Hats. Stayed because its genuinely fun.
If you want a truly social experience:
Phantasy Star Online 2: New Genesis
Phantasy Star Online 2 had 10 years of content all free, then it was rebooted into New Genesis which is super new player friendly with it's own 4 years of content.
It an action RPG, real time combat, quests with fully voice acted cutscenes, it features an open world where you can jump in and help others mid combat at anytime.
XP is shared, it has an max invite party size of 4 but with proximity combat you can early find yourself in a group of 20 players all sharing XP and working co-operatively.
The only microtransactions are cosmetic. It has the most insane emote system you can find outside of IMVU or Second Life, you can build your own home (Which counts as a cosmetic) or raise a pet just for fun.
Stay out of the casino I beg you 
It's a Sega game so it has a colourful casino.
Roleplaying is wide spread as the various cafe's, shops and gardens all exist solely to be areas where you can sit and converse.
It wasn't a waifu game, the male characters have the same level of customisation as the females...
But 80% of the players are female.
New Genesis wasn't well received at launch (It wasn't necessary) so Sega eventually made PSO2 a separate download from the new base game as back then people just continued playing PSO2 Classic. Completely ignoring the NG content.
That didn't work, PSO2 proper always pulls in a player count well into the thousands, yet New Genesis's pulls in around the same number now-a-days. Average peak is 1.5-2k on steam, bare in mind that's not factoring in the Xbox, Playstation or stand alone launcher player count.
It's never quiet.
You can also sync your player data to Playstation/Xbox account for free. So it's cross platform. My girlfriend has it through the Microsoft store whilst I use steam. We play together regardless of the storefront differences.
It has Full controller support too.
Can you guess which one I'm addicted too?
Please help I've had four accounts over the last decade on both NA & SEA It dragged me back in so many times Spiral Knights is the better choice but Sega has my balls in a vice run whilst you can.