I'm a piper (not military, but I do do military reenactment as a WWII Argyll and Sutherland Highlanders piper). My favorite tunes are as follows:
4/4 March: You guessed it...Scotland the Brave. There's a reason why it's played so much I say! I like playing it in sets as follows: STB/Lord Lovat's Lament/Braes of Killiecrankie/STB. Or Cabar Feidh/STB/Haughs of Cromdale/Col. Sinclair
2/4 March: Many good ones, I particularly like Balmoral Highlanders, Inverness Gathering, and Lord Alexander Kennedy
6/8 March: Atholl Highlanders. I can play the full 6 part version though I often play an adapted 2-part version.
Retreats: I like Colin's Cattle, Lochanside, Highland Brigade at Magersfontein. I couldn't care less for "Green Hills of Tyrol".
Strathspeys: Duke of Gordon's Birthday, Miss Drummond of Perth, Marquis of Huntly and Miss Ada Crawford are among my favorites.
Reels: I love Sandy Duff, also Reel of Tulloch and Miss Proud.
Slow Airs: The Flowers of the Forest, The Return of the Unknown Soldier and the Sands of Kuwait are among my favorites.