I'll admit that I've never understood why the Capellans were treated like this big badguy when they got curb-stomped left and right more often than not (I've always scratched my head as to why the FS didn't concentrate on the Dracs instead). Least starting with Sun Tsu a guy like me can finally see the Cappies as a worthy adversary.
The anti-cappellan thing stars in "Sword and the Dagger". When Max Liao replaced Hanse with a fake Hanse. It incited Hanse's terrible hate of the Capellans, Even though he disliked and even hated the Dracs, Ian having died at Yorinaga Kurita's hands. But it was a very personal view of Hanse's sense of right and wrong when you erase a person's life,m experiences, mind and persona. I always felt he figured there was some lines you just, don't ever cross. And yes I do remember Joshua Marik thank you very much.
But back to Units.
1)Lyran Commonwealth= 10th Lyran Guards, Even after their near destruction at Dromini they always seemed to return better, stronger, and with a greater strength of purpose.
2) FWL= 5th Oriente Hussars. Lt. James T. Henderson. Pilot of an AWS-8Q Awesome. plays a game poker wich remains unfinished, he leaves behind a 'Dead Man's Hand" of 8s and Aces. The 5th were always stationed on the Trinity Worlds. They tended to intermarry and breed with the people on those worlds. This was done, iIrc to maintain a sense of unity within the FWL.
3) FedSuns= Davion Guards; The Best of the Best, the unit every FedSuns mechwarrior aspires to be in. Best Equipment, Best Training, Best of everything. No real flaws, no real want for anything. Except they are kept in reserve an awful lot.
4) DC= Ghost Regiments- The 9th are my favorites, BA enough to go toe to toe with the Cabelleros, and thye didn't have the proverbial Katana up their rears that most Drac mechwarriors have.
5) CapCon= I always kind of ignored them. I don't ever play as Cappies so I somewhat limited my view of their units for good or ill. So I'l lpass on this one.
Addendum: For long serving Merc units, my favorite is Snord's Irregulars in the Lyran Commonwealth/FedCom/Lyran Alliance.