My group uses the SPAs as outlined in the Formation Bonuses... a healthy mix of every type for most battles, though (all of us on my end) tend to favor the Recon + IndirectFire/Artillery lance combos ...which has often lead to thrown dice and foul moods.
Since only two of us actually own the ASC, we tend to just stick to the Formation bonuses. We've occasionally sprinkled in uncommon/unattached SPAs not available through the Formation bonuses (such as Marksman, SharpShooter, and Melee-oriented SPAs) but primarily assigned them to specific, skilled pilots (1 per formation as Lance/Star, either the lance/star leader or specialist).
We also use a few home-brew Formation types geared towards Ambusher-types and weapon specialists, but haven't really play-tested them thoroughly.
As far as making my peers sweat? My pair of Mars Assault Tanks [HAG] would certainly qualify. Granted they are slow and expensive, their presence on the field has proven devastating if taken lightly or ignored. Coupled with some of the above-mentioned SPAs, they are outright mean.