There's a part of me that just wants to say the top row of Megatron, Starscream & Shockwave. All of them are powerful but have BIG personalities/egos. Starscream would probably try a betrayal and mess up everything.
List 1
$5 Megatron
$4 Soundwave
$2 Ravage
$2 Laserbeak
$1 Buzzsaw
$1 Rumble
Hmm... I went a little too heavy into the "cassette" Decepticons, but picking Soundwave is a must because he's one of my favorites. The cassettes aren't necessarily the strongest, but with enough of them, they can be very versatile.
List 2
$5 Megatron
$4 Soundwave
$3 Thundercraker
$2 Laserbeak or Ravage
$1 Rumble or Buzzsaw
Okay, apparently I have to take Megatron and Soundwave. Although Thundercraker (the blue jet) isn't the toughest Decepticon, he's affordable for some air power. Laserbeak and Ravage are pretty tough, stealthy and reliable; either would be an asset. Buzzsaw might make a acceptable replacement for Laserbeak and Rumble is a wildcard but useful.