My childhood was filled with Atari 5200, Apple IIgs, and Gameboy.
Atari 5200:
Pitfall II. I'm not sure how obscure this title is, but I only ever hear its predecessor mentioned, not the sequel. In fact, I think this might have been an unlockable in the newer Pitfall-series game for GC/PS2/Xbox, so I might have to pick that up after all. This game was great because I thought it was the only Atari-age game with an ending (I hadn't heard of "Adventure" back then) and so it was fairly revolutionary to me. It took me a year to get to the 2nd cavern (stupid balloon!) but once I did I beat it fairly quickly. The Atari 5200 controller was 95% of the difficulty of any given game for that system.
Apple IIgs:
Tass Times in Tonetown What can I say, it's ultra-tass! This was a hybrid text/graphical adventure game where you're shot into an alternate reality and the focus of the game is "blending in." You can buy weird clothes, eat odd food, get chased around by a talking aligator that runs a pet store, buy a pet "blobpet" (that actually does things in the game), and basically try to rescue your grandpa. Fun for the whole family! This game really turned me on to the whole idea of PC gaming vs. console gaming, and easily showed off the power of the Apple IIgs when compared to the FAR inferior IBM-compatible version. God, IBM-compatible... doesn't that just bring back memories of the olden-days?
Gameboy:
Boomer's Adventures in Asmik World This game had a pink dinosaur on the cover. You went around a maze digging stuff up, like the compass the would point to the buried key that you had to find and dig up to access the stairs to the next level. You could dig a hole, an enemy would fall in, then you would bury the enemy. You could eat ice-cream cones to gain "freeze breath" or something similar to get fire-breath. It seemed really cutesy at a basic level, but it was extremely hard. Each level had ~3 different random locations the key could be buried in. So, you could memorize these 3 locations, but you never knew which specific one it was in when replaying that level. Also, the boss fights in this game are some of the hardest you will ever run across. When you got to the top of the tower of levels... well, let's just say this game played a familiar trick.

This is actually one of my favorite Gameboy games. I got it as a birthday present (normally, I don't buy games about pink dinosaurs.

)
Also on Gameboy:
Gargoyle's Quest To this day, I still feel that Gargoyle's Quest had the best Gameboy graphics. I played a ton of Gameboy games back in the day because this was my only system (I wasn't allowed to have a NES so it wasn't until years later that I picked up a SNES). Since then, I have tried out just about ever Gameboy game made via ROMs. Gargoyle's Quest was definitely a Gameboy game ahead of its time in the graphics department.
Jam it back in, in the dark.