While I'm partial to the 349th Royal BattleMech Division (The King Henry Division) - I get to chose which Henry depending upon which mood I'm in - and the 341st Royal BattleMech Division (The Black Prince Division), I have to give the nod to the 309th Royal BattleMech Division (The Black Charger Division). The original Black Charger Tank Division was an elite unit all the way back to the early 25th century when it was in HAF service, notably holding Terra Firma for two months against overwhelming CCAF forces, which included naval support, during the Capellan's invasion of the Hegemony in 2409.
While the CAANs aren't division sized, I am also fond of the 29th Royal CAAN Regiment, as it was one that I used in a few RPG and BT campaigns way back in the day.