Let me start by saying I have likely 300+ colors and only missing a few of the blues and the darker neutrals so it is REALLY HARD for me to narrow that list down to 12-18 because sooooo much depends on a) what colors YOU like, and b) what PAPER you are trying to match it to. The BG's for example have a WIDE range of values from very blue to lots of green so to pick a couple from that list is purely based on what I PERSONALLY would choose to start with, and I've included some very light E's in here because I almost always color faces so those colors would be essential for ME!
So here goes!
E000, 00 (good for flesh, use R20 for cheeks)
R20, 22, 24 (nice coral reds)
G40, 82 94 (3 of my current fav greens, also really like the YG's)
YR31, 23 (nice golden yellow)
BG000, 01, 05 (because i'm not fond of pure blue!)
E30, 33, 35 (nice warm browns, good for hair!)
colorless blender and refill
Note that there are also no purples in there (V's or BV') but if I were to add those in it would be:
BV000, V12, V15 (these tend to be on the greyer side, because that is what I like)
I have also avoided pure yellows because they can be look too bright (to me!) so I opted for the YR's in my must have list. But, if I had to pick some yellows they would be:
Y00, 11, 15
Another good thing to know is that any of these combos could be lightened or darkened by adding pencil crayon so just because you don't have a full range, don't despair! Get some pencil crayons, and OMS (Gamsol, Sansodor, etc) and you can EASILY add depth and highlights right over your copics after the copic ink has dried. Also, these colors may not match the paper you want to use, so again, you can use those pencils to vary the shade of the copics. If my fav's are not your fav's, you should also check out the combinations that Flourishes has put together, because I think they have done a GREAT job and I LOVE the combinations they have come up with. You can find those here.
Experiment! It's only paper!
So that's it! Linda's choice (please don't make me choose between my children or my dog!...LOL!)