1. Heroes of the Inner Sphere - Career mode is a must and the extra stuff on top is great.
2. Dragon's Gambit - Handled really well and a real fun Endgame Challenge. The Longbow is a one trick pony, but its a fun one.
3. Solaris 7 - Have not played the actual campaign yet, but the Arena missions and extra stuff added some much needed life to industrial zones.
4. Rise of Rasalhague - Really good lore integration into the campaign. Be prepared though, the short time between missions really gets you if you don't have a few spare assault mechs.
5. Kestral Lancers - A good campaign but of the campaigns its the one I like to replay the least, its a bit one note. I think its actually a bit too generous and can accelerate the "up and coming merc company" feel of the early-mid game a bit too quickly.
6. Call to Arms - Its honestly pretty lackluster and the hatchetman is meh. The added biomes and finally giving MW melee is awesome though. Its not bad, just the least good.