Rec
The Gap Between, McStories, McCoy/Chekov. This story makes the pairing not only plausible, but believable, something which a lot of the ship doesn't do. Readers of McChekov are often expected to just take it for granted that the two have been in a relationship, or that they would be spending lots of time together (and what the hell, it's pretty to think about so we go with it). But this is one of the first stories that actually makes sense of the relationship.