THE LEGEND OF ZELDA : ORACLE OF AGES/SEASONS / Nintendo / GBC
Oracle of Ages and Oracle of Seasons are two separate games that share the same engine, but take place in different worlds. They're interconnected in an interesting way not really seen before - completing one gets you a password that you can start a new game with in the other, and this is the only way to reach Best End and fight the real last boss of the game. Through the course of the game, you can also pass certain items and rings (bonus collectibles that give Link various stat boosts and such) to yourself between games via a password, or trade rings with other people using a link cable.
This game was the first outing with the Zelda series for Capcom subsidiary Flagship, and development of it actually began as a port of the original Zelda to the Gameboy Color. They obviously kept and used a lot of the material they generated while working on that - you'll notice familiar locales and powered-up versions of enemies from that game (as well as those random old men that live in the dungeons and dispense advice, and those old bastards that jack your funds when you burn down their front doors). On the whole the games don't do much that is original - they use the same basic engine that Link's Awakening did, and have characters and items from various other Zelda games sort of slapped in there as well. They do have some new characters, however, such as a boxing kangaroo and a flying bear (Frying Bear?!) that give you rides and trash up enemies when you call them. The main "gimmick" of Ages, which is time travel, is similar to what was done in Zelda 3, but the gimmick of Seasons is new and fairly inventive - the villain of that game has knocked the seasons out of whack, so now they change at random times and in random areas. The current season affects the terrain - for example, in summer vines may grow on walls allowing you to climb them, and in winter packed snowdrifts may accumulate between ledges and give you a platform to walk across.
The graphics are about as nice as you can get on the humble old Gameboy Color, with a surprisingly good and evocative use of color (particularly in Seasons). The game also circumvents it's graphical limitations by using more cut-scenes (which are pretty nicely drawn) than Zelda games usually do. The soundtrack is serviceable, nothing really special but it does the job and some good tunes from the previous games make a re-appearance here.
Like Link's Awakening, these two don't really add new elements or take the series in a different direction, but they are very polished, playable and generally well-executed. The dungeons should not disappoint, as they are among the more challenging in the series, and between the two games you get 16 of 'em. I think these are two of the best games for the Gameboy/Gameboy Color and certainly worth a look.