I think the First Free Worlds Legionnaires were actually better than the 1st Free Worlds Guards in the 3040s-3060s. The Guards get put on a pedestal, but the First Legion gets nods for successfully being in several minor campaigns in the 3040s. Combine this with the Legion focus on combined arms. I can't remember who they fought in the 3060s but when Liao forces did land and fight them in a minor raid they smashed the Liao forces quite easily.
The First Free Worlds Guards, for all their acclaim and having the best of everything, seem to struggle to win. They seem to suffer a string of defeats, including during the Great Refusal. Best of everything, still struggle to win battles. It's entirely possible that they've gotten a lot better in the recent eras.
For me, for the FWL it's 1st Free Worlds Legionnaires during the 3040s-early Jihad era. Then possibly 12th Atrean Dragoons (saved Kristen's Krushers, became the core of the anti-Blakist resistance all the way to the liberation of Terra) as runner-up. Then we go to the First Regulan Hussars. The strike mark against them is the lack of Level 2 tech in the pre-Jihad eras. The FWLM intentionally kept them down and low on the logistical priority list.
Late Jihad era (after the breakup of the FWL) and beyond, it's probably the 1st Regulan Hussars in the number 1 slot. As they maintain that intense brawler fighting style but also combine it with receiving a lot more support, newer equipment and so on. Hard to argue with Regulan lethality once it gets unleashed and allowed to advance to its full potential.