Star Wars Galaxy of Heroes.
Marvel Future Fight.
Yu-gi-oh Duel links.
Fantasy War Tactics.
Ads are virtually non-existent there are Micro-transactions but all 4 are easily playable without spending a cent. For example, I got to Vibranium League(highest PvP Bracket) in Future Fight as a free to play and have done all the PvE content.
I am rank #500-600 in PvP in Galaxy of Heroes out of 1.1million players without ever spending a cent.
The other 2 you can do well in easily without Micro's either, micros for them is moreso a speed up/convenience anyway as opposed to increased power.