Out of all of the various games I have on my iphone, the ones I keep coming back to are the Cave bullet hell shooters. Dodonpachi Resurrection especially.
I keep playing them on the lowest difficulty, and its still plenty challenging, yet it's rather fair. Resurrection's Smartphone mode really goddamn fun. Essentially, your ship, whichever of the three you choose, switches between a wider normal shot, and a laser, which you absolutely NEED to use to block enemy lasers. At the top, you have the SM gauge. As you kill more enemies, the gauge leans towards the S, and when it's maxed out, it boosts your shots, letting you cut down through swaths of enemies and deal even more damage on minibosses and bosses. The enemies killed when in this mode drop TONS of point items. The downside to this, is that your Hit Multiplier starts going down. You increase the hit multiplier by grazing bullets, which increases the M side of the gauge. In this state, your multiplier increases dramatically, as well as your hyper gauge, but your shots aren't powered up, and enemies aren't dropping all those wonderful waves of point items.
Essentially, you have this give-and-take system when you need to shift between the two modes to get a massive score, as well as to completely decimate the enemy.
The big thing that keeps me playing, though, is trying to enter the Second Loop, which allows you to fight the true final boss of the game, Hibachi. Only thing is, you need to collect all these Bee Medals in order to do so. These little bastards are hidden in the stages, and you can only uncover them by hitting the spot where they are hidden with the head of your laser. Doing so uncovers them and let you collect them. Oh and there are technically two second loops. The Open version requires you collect 35 of these Bee Medals, and takes you up to Taisabachi, which is basically the first form of the true final boss. Collect all 45 (which requires you be on the secret path, which adds two more medals to each level), and you get the Closed version, which lets you face Hibachi, who is an utter bastard boss that will destroy you in a gigantic wave of bullets. Naturally.
And if you beat him? You unlock him as a playable character.
And he's goddamn amazing.