Meling and Syaoran
Meling and Syaoran, like this wasn't obvious in the first two seasons.  Not too much can be said cause its pretty self explanitory.  The most i can explian that you might not already know is that Meling is actually Syaoran's cousin AND fiance.  In Hong Kong (before the CCS story even starts) Meling and Syaoran made a promise that if he fell in love with anyone that they would break off the engagement.  In the third season Syaoran calls Meling and tells her that when he comes back for winter vacation he has to tell her something.  Meling being the impatient girl that she is comes for a little visit.  Meling, Syaoran, Sakura and Tomoyo encounter a little incident and after its all over Meling hears Sakura call Syaoran "Syaoran-kun" instead of "Li-kun" and Syaoran calls Sakura "Sakura" instead of "Kinomoto".  Aparently the only people that call Syaoran "Syaoran" are his family and Meling (not even Wei his butler calls him by his first name) so this sets Meling off right away.  Syaoran has Meling over to his house and she asks him what he wanted to tell her.  He starts of by saying "remember the promise we have about..." she knows right away and when he has finished his little speech she says "Its Kinomoto isn't it"  Syaoran turns a nice healthy red. and Meling says (with quite control) that the engagement is off then she runs to tomoyo's house and crys about it, saying she doesn't know what to do cause she loves Syaoran and she wouldn't let anyone else have him, yet she really likes kinomoto too, and she can see why Syaoran is in love with Sakura.  Then in the second movie she and tomoyo join forces in getting Sakura and Syaoran together.