Copics are soooooooooooo awesommmme!  If you didn't know.  I can't even imagine life without being able to refill the markers.  It's just...not fathomable.  I've developed a special fondness for a few colours, even so far as to have them be my 'go to' colour for a certain kind of object.  Huzzah for favourites!  

An excellent comparison of Copic markers and Prismacolor, if you're new to the medium and have questions:

B12 "Ice Blue"
This one is my sky colour.  It's a great light, bright blue, cheerful and clean.  I find it layers over itself alright, but not strongly.

Y23 "Yellowish Beige"
BEST BLONDE COLOUR EVER!  And so versatile!  You can use it as the dark shade on a white-blonde person, or as the dominant colour, with darker tones to accent.  I can also use it to give a 'gold' object a more realistic tint, instead of just using bright yellows.  It doesn't layer well.

G82 "Spring Dim Green" and G94 "Grayish Olive"
These are my plant colours.  Great, not too bright foliage tones.  I've also begun to use BG18 "Teal Blue" in the mix also, for trees, as a shading colour.  All three colours layer adequately.  

E71 "Champagne"
I bet there is a smidge of this colour in EVERY drawing I do.  It is so versatile!  Hair, eyes, rocks, wood, trees, clothing, you name it!  It's a great neutral that isn't too dead, like the warm and cool grays can be.  It layers alright, depending on your heaviness of hand for the initial layer.  

E41 "Pearl White"
Another colour that is probably in every drawing.  This is what I shade warm white with.  (Cool bright whites I shade with C1.)  It's also a good base for believable white-blonde hair, and for eyes or teeth.  Also excellent for clouds!  Gotta put clouds in that bright B12 blue sky.  Doesn't layer very well over itself.  

E04 "Lipstick Natural"
My newest favourite!  Although this colour actually sucks a lot as a "lipstick natural" colour, it's too deep and too brown for light skin (but I bet it would like nice on a darker skin tone!)  I use this for rocks, trees, hair, clothing.  It's a miracle colour, it adds life, but it's not overpowering.  I'm getting pretty desperate for a refill, at the rate I'm loving it.  Layers over itself alright.

There are more, but these tend to get the most use.  <3