Stargate Atlantis/Stargate SG-1: John Sheppard/Cameron Mitchell.
Frostfire’s characterization is always, always amazing. It’s like she lives in Sheppard’s head. This is the perfect crossover. It’s set in Afghanistan pre-Atlantis (pre-SGC for Cam.) Seriously, read this even if you don’t like seeing Shep with anyone else or if you don’t know who Cameron is. It’s a fantastic read. John is brutally realistic and the Afghanistan backstory is just how I think it should be.