For me:

1: Combaticons - they all turn into military vehicles, which seems to make a lot of sense in an eons long civil war. Add's great play value to setting up toy battles too

2: Stunticons - first combiner group I ever completed as a kid

3: Aerialbots - a bunch of Autobot jets added more storyline options for the cartoon and more play battles for the toys.

4: Protectobots - Hot Spot rules as a toy! In fact they were all pretty good.

5: Technobots - only completed my collection last year. Great to see a team made up of totally cybertronian vehicle alt modes. And Scattershot in my opinion is about the best looking alt-mode commander.

6: Seacons - love the fact that the 5 smaller ones all had a Targetmaster mode and that they were interchangeable in gestalt mode so anyone (bar Snap Trap) could become the gun.

7: Terrorcons - Hun-Gurr looks pretty cool, but having a figure like Blot on the team definitely puts them last.