i havent had games on my phone in forever but i tend to lean towards jrpgs or mmorpgs so my top picks would be
Unison League by Ateam
Alchemia Story by Asobimo
and
Sword Art Online: Integral Factor by Bandai Namco
there not super popular but theyre easy to pick up and imo pretty fun to play
i will say thay unison league in particular has a weird growth curve where unless theres an ongoing event (especially collabs) leveling up takes a while and then of your not balancing your skill tree properly especially across classes that can mess you up in the long run depending on what class you want to master and then bosses after a certain level. the latter at least doesnt really hit till about level 50 or when you've gotten past either regovas or the level after the giant wind tree (the light kingdom iirc) but besides that its pretty smooth sailing.
alstory and unison leage do have some buff isues with big group bosses tho so it doesnt hurt to try to make friends so you can party up or just find strong players who dont mind carrying in the open lobbies which for alstory at least isnt too hard since server switches kind of make up for social stuff so soloing is pretty breezy
easiest one to progress in tho is sao:if and even tho theres a part that'll like try to make u guild up there's literally no obligation so you can like join the one it auto applies you to and then you can pop right out and go back to soloing especially since guilds can be hella inactive and folks only really do the big raid bosses if theyre tough enough and its usually solo players or strong players whove been together a while so yeah.
also if you're into in game events all three tend to have stuff periodically and unison league i specific does collab events (usually related to other games made by Ateam) so youll see like Hatsune Miku, Evangelion, Touhou, AOT and other relatively popular franchises so lots of collab monsters, gear and other stuff with like hella useful buffs (Miku collabs always have great healing ones for example)
but apologies for the longwinded reply and i hope you find something you like 
