I find I tend to use the same groupings a lot, to the point I almost ditched RPGO and made my own set of classes inspired by those specialization combinations.

Ones that have worked well for me, aside from those close to the base classes:
- Sniper Rifle and Ripjack (I use Sniper, Assassin, and Skirmisher): the ripjack's grapple allows the sniper to quickly post up on elevated surfaces, and also gives a melee attack if enemies get too close.
- Shotgun and Shield (I use Assault, Scrapper, and Protector): pairs especially well with Juggernaut and Blast Padding, functions like a normal assault except harder to kill.
- Pistol and Gremlin (I use either Survivalist, Medic, and Combat Medic for a dedicated healer; or Gunslinger, Hacker, and Code Breaker for a cyberwarfare expert): Personally I'm a fan of pistol primaries, and tend to use them for bombers as well, but they seem especially fitting for more technical roles to me.
- Cannon and Gremlin (I use Gunner, Machinist, and Operator): a dedicated anti-mech unit!

What are some of the groupings you use a lot of?