Disclaimer: This post contains D20-based crossover shenanigans.
Best legion is the 180th. They have an attached ARC trooper, two...three...about six Jedi (including big-name figures like Revan and Bastila), commanded by a Grey Jedi with more dark side points than Darth Vader. Resume includes defending the Jedi Temple against the Seps at the Battle of Coruscant, capturing the planet Talasea against 4-1 odds, surviving Order 66 by fighting off a full legion-sized grouping of the 501st+support fleet immediately after taking the planet, repulsing numerous attempts by the Empire to take the planet, seizing the largest military base on Coruscant during the Republic/Alliance recapture of the planet from the Empire, discovering the Empire's plans to build a Pulsar Station, resigning en masse from the Republic, joining forces with the Mandalorians, and declaring Talasea a haven for any forces from either the Republic/Alliance or Empire who are disillusioned with their side.
Disclaimer: This post has contained D20-based crossover shenanigans.