I like the PAMI the best. I feel they are the most versatile, with the most weapon and equipment options. In one platoon you could have different squads with either WASP packs, mini-platform weapons, reliant platforms or other assets, with a Marauder, on LA-50 Sprites, or with various weapon upgrades. And the platoon command can be in Marauders as well, without being a Marauder platoon. They can also use drop capsules like the Exosuits and Marauders.
My impression of the others:
LAMI- Their advantage is numbers and emplacement assets but they are vulnerable to being too static and being overrun. Firezone restrictions are a negative too. They're still fun if you like the movie.
Marauders or Exosuits- A lot of firepower concentrated into fewer models. They lack the "in game" mobility of the PAMI since none of the squads can get hover/18" through WASP packs or Sprites.
Roughnecks- This is really more a unique character list to add to a PAMI platoon rather than a separate type of organization. I consider them good flavor and fun if you're a fan of the Roughneck Chronicles series.
Pathfinders- No opinion yet. I'm still waiting on that army book to show up in the mail..
Despite how effective I think they are, I still collect all of the above for flavor purposes. They're not all table ready yet, but I've got a Marauder platoon, 2 PAMI platoons, the Roughnecks with a CHAS, a small PAMI WASP platoon, 2 LAMI platoons, and an Exosuit platoon. I'm just starting on Pathfinders.