i've just downloaded bought about 2600 gameboy/colour games and i'm guessing around 90% of them are absolutely terrible.

Can anyone help me root out the good ones?

So far i've found these decent ones:

Adventure Island I and II
Pokemon Games (obv)
Legend of Zelda
Game and Watch Gallery 2
Game boy gallery 3 (same as above but in colour)
Gameboy Wars (older advance wars but japanese tho -_-)
Battletoads
Mega Man II - V
Rockman lol
Lemmings (haven't checked if this is anygood actually)

Super Mario Land 3: Wario Land   (WL2 and WL3 are pretty good but WL1 is just really really good)

Survival Kids is a pretty cool game I had as a kid, it's essentially a prequel to the Lost in Blue games on the DS. You're stranded on a tropical island and you run around eating berries and combining bits of island garbage to make tools. It was pretty deep for it's time! All of the wild plants have randomized effects every time you start a new game (kind of like a roguelike), so you have to test them out to find out which ones are safe and which ones make you sick. Also the music is catchy as fuck and will stick in your brain until it drives you absolutely mad

here have a youtupe: http://www.youtube.com/watch?v=eQet3NwtHO4

i like Dragon Warrior Monsters a lot as a kid

Oh man, that's so weird.  I put batteries in my GBC the other day, and have been playing some games on it.

Metal Gear Solid was pretty good, from what I remember. 
Army Men was also great.

Super Mario Bros 2

Yeah, Super Mario Land 2 is easily one of my favorite Mario platformers.

It's just so good.

Survival Kids, Mole Mania, Chu-Chu Rocket, Pokemon Pinball, and all the Hamtaro games. those were what i always played and they were really great yeah i loved the Hamtaro games; i own every single one every made, haha. they were great when i was a kid and after playing them again, they still are. they were adventure games where you had to know how to speak in "Ham-Ham language". so the options you could say to other hamsters were limited until you expanded your vocabulary by talking to more hamsters. they had some good battles and clever puzzles, and were way better than i had expected them to be. you may be doubtful that you would enjoy the Hamtaro games but try them and you might be surprised! :3

P.S. interesting fact: did you know the Hamtaro series (and the design of the main character, the hamster) is based off of a 1990s adult cartoon called Oruchuban Ebichu ("Ebichu the Housecleaning Hamster")? it is the most adorable and perverted show ever and it's amazing. i watched the whole thing; i love it.

i cant seem to find chu chu rocket, mole mania and metal gear 

but anyways thanks for the rest of the recommendations i will proceed to play for hours on end :P

Crystalis.

It's very good if you like games similar to Zelda.

metroid
kid icarus
marioland 1 and 2 + warioland
Mole Mania
Donkey Kong(it's not the old one, it's actually a really long game that uses the whole premise of DK but expands on it)
James Bond 007

The only two I can think of that haven't been mentioned are Legend of the River King (FISHING RPG but it's not the absolute best game, I just always liked it) and Monster Rancher Battle Card (the card game part of it is really neat but every other aspect sucks, still you can manage through the other junk without it getting in the way).  Oh and if you do like card games, the Pokemon Trading Card Game...game is pretty fun.

I thought there would be nothing on this list but I was wrong, I really wanted to try that River King and Survival Kids when I was a kid

yeah Dragon Warrior Monsters was ok but really Dragon Warrior with even less of a plot and random dungeons is an awful idea so maybe you guys are only remembering it as good

how much stuff is actually good game and exclusively for game boy seriously there must be able to count all the games on your fingers

I used to have one of those cartriges with about 20 games on and the real stand out game, in so far as the time you could spend with it, was Bubble Bobble. I heartily recomend it.

Scooby Doo. It was so so great.

I had a Pokemon romhack of Survival Kids as a kid that I thought was a Japanese pokemon game that just hadn't been translated yet

Tetris is probably the greatest game ever get that