Even with its buggy programming and weird tutorial decisions, Marvel Puzzle Quest is great. This iOS/Android free to play puzzle game offers a variety of characters, levels, upgrades and gameplay systems that made me play it a ton. The free to play hooks aren't too bad and don't remove from a good experience matching gems and being the hawkeye.
Through multiple missions on the world map, you fight various enemies with different powers and go through a little marvel story. Levels have a few things they can grant you when you win, from currencies (there are two of them (and one is a premium currency, although you can win it in-game)) to comic books (used as characters or to level-up your characters). Winning a match only gives you one random reward, so you're encouraged to try levels multiple times. The tutorial stuff is pretty busted though, you get infoboxes about character abilities that you might not have in your team, you're forced to spend premium currency on characters you might not want in the tutoriall and some popups come back everytime you do a level, for no reason. How about you plan this better? Remove useless messages and don't force me to waste my currency.
Oh and while I'm at busted stuff, the music/sound randomly dies until you reboot the game. Not a design thing, but still annoying.
You make a team of three superheroes to go into each level. If they were injured during the last fight, they still are missing that hp. It goes up slowly over time, acting as an energy mechanic. It's not too bad because playing skillfully will make your characters take less damage than if you were playing really 'badly'. You can spend healing tokens - or something - to bring them back up in shape instantly, and you can buy these tokens if you don't want to wait for your team to go down. You also can have more than three heroes total, so even if your roster isn't in top shape, you can just switch around.
Each character has multiple stats, HP, damage per gem (the amount of damage they deal when you match specific gems), and multiple skills. You can raise their level up to the combined level of their skills by using the 'free' currency, and you can raise skill levels with comics or currency. To unlock new skills, you absolutely need to find the new comics associated with them. Skills range from dealing damage to destroying random gems to turning gems into attack gems (that deal damage every turn to the enemy) to stunning your foes, there's not a great variety of heroes if you're not willing to pay early on, but you slowly get to build the team you want. Maybe you should have more choices at the beginning. I'm not a fan of Iron Man myself, but he's kinda strong, so I'll use him.
I really love the systems they've put in place for the core game. You build up AP of multiple colors by matching these gems, then you can spend this AP on same-colored skills that your characters might have. Whenever you match gems, you also attack with the associated character and that puts your character in front of the party, so it's the one that enemies target. If you plan your team badly, some characters might never attack (because they deal the less damage with all gem types compared to your other party members, so why would they attack?) or you might have certain colors that aren't used.
Random enemies such as soldiers and robots just accumulate AP over time and unleash attack gems on the board, if you don't destroy them in a few turns, they activate, dealing damage to you, turning gems to other colors, healing themselves, the list goes on. Bosses will match gems and use abilities as you do and they're much more dangerous.
I love puzzle quest and I love grinding, I love leveling up abilities and making crazy gem combos randomly, I wish this would've been 9.99$ instead of being free, but I'll keep on playing it. Hope they fix the bugs!