I like ZAS.
The mechanics are fairly basic. You have two option pods which you can either have close to you for a concentrated shot or spread out. The A button switches between the two formations.
ZAS is one of the most beautiful games on the Game Boy.
It uses the slow screen refresh to mix frames allowing for beautiful backgrounds and weather effects.
Aside from looking amazing, the game also has memorable boss fights.
The stage 3 boss in particular is devilishly fun.
There are bullets being fired on both sides of you creating a sort of road. Enemies are coming out from the boss and shooting at you but if you kill them they fall faster. Kill them at the wrong moment and they block your path killing you via collision.
It’s very frantic. You have to be careful about when to shoot them and when not to, all while also trying to shoot the boss behind them.
The last boss fight has a surprising and potentially frustrating gimmick.
You fight against two monsters that have the ability to resurrect one another so you need to either kill them at the same time or kill the second immediately after the first.
Screenshots don’t do it justice. Stages 1 and 3 look stunning in motion.