When my nail polish was wet the colour was almost identical. However when it dried I've noticed that Chanel Graphite tends to catch light more. The glitter particles also appear to be larger so it sparkles more.
Chanel is also tends to be more opaque.
OPI Number one nemesis is still a good dupe, however it is not as multidimensional as Chanel.
So overall Chanel wins. However if you don't want to pay more than $20 for a nail polish or can't find Graphite go for OPI Number One Nemesis. The difference is so subtle that no one will be able to tell whether you are wearing Chanel or OPI :)
Both nail polishes performed well in terms of wear, colour opacity and drying time.