If I had to choose the most unique or most insane GBA game, I would likely cast my vote for Boktai: The Sun Is In Your Hands, the Konami game that is built around the use of a light sensor. Leave it up to Hideo Kojima, creator of Metal Gear, to come up with some really bizarre game ideas. I suppose if the man behind one of their most successful franchises tells them to buy a bunch of light sensors because he has an idea for a game, then Konami is not likely to tell him no. Whatever the case, I am glad it got made because it is a really interesting game.
Basically everything in the game is based around the use of the light sensor, which means the player is actually required to go outside, or at least use or strategically placed black light. I imagine that Boktai is responsible for more than a few sun burns, possibly a couple of extra cases of skin cancer, and in the case of people using black lights, a few partially melted handhelds and cartridges. I wonder if it was just Kojima trolling gamers by forcing them into the sunlight or if it was just a side-effect of the wacky ideas he wanted to put into the game.
The goal is pretty simple, there are some evil immortal vampire guys, and the only way for them to be killed is for the main character to drag their coffin to a special area and shoot them full of light. The game is an isometric action/adventure/stealth game that centers around using light to defeat enemies and move through dungeons. The player's gun is capable of stunning or killing ghouls, but it requires a charge of sunlight to work. It can be recharged by absorbing crystals or holding it up in sunlight. Of course, sunlight can be hard to come by, as it requires the light sensor to be detecting sunlight, and the player needs to be in an area in the game that sun reaches.
When outside, if the sun is shining, then the player can recharge his gun and lesser enemies such as ghouls will die instantly. When inside dungeons, the player will need to rely on sunlight coming in through windows and things like that. There are also recharge stations that don't require sunlight, but the available charge is dependent on how much sun the sensor has gotten. With limited charge for the weaponry, it is important to use stealth to more efficiently pass by ghouls, and it is a Kojima game after all. It is even possible to knock on walls to distract enemies.
The player only gets one gun throughout the game, but it can be modified in a number of ways, so there actually is a good deal of variety. There are single shots, spread shots, melee attacks, automatic fire, and other methods of attack. There are also a variety of upgrades that do things like extend the amount of sun charge that can be held at one time. This, combined with general flow combat gives it a similar look and feel to Dark Arms for the Neo Geo Pocket Color. It is quite a bit of fun, and there is an odd joy in dragging a coffin into a sun-powered laser cannon.
I don't know how successful Boktai was, but it must have done alright if it spawned three sequels, two for GBA and one for DS, though the last GBA one was never released outside of Japan. Personally, I find the game to be interesting and charming, and generally just pretty fun. It is one of those unique titles that can only be found on the GBA with weird cartridges, like Drill Dozer and Wario Ware Twisted. It's just an experience that can't be emulated. Sure, the game itself can be, but pressing a button to adjust the light level just isn't the same.