I mean realistically it's impossible to rate new era regiments against old era regiments so in terms of the 'best' old era regiments in their primes you could say for EU it'd be between;
91st, 92nd, 17e, 84e, 77y, Nr.24 and 14e.
Personally out of the ones above due to longevity I'd rule out 77y even though in their prime they drew with the 91st and almost beat them on another occasion. I personally believe that once ruling out 77y it goes something like this (considering the amount of matches all these regiments played and their competitive records in their prime); 91st, Nr.24, 92nd, 14e, 17e and then 84e.
In terms of 'New era' regiments you have a few like 66th, K-KA, 82e and 15thYR. I believe in their primes it'd go 66th, K-KA, 82e and 15thYR. But this is a very hard one to call between 66th, K-KA and 82e as each in their prime periods have won leagues or regimental groupfighting tournaments. I know K-KA could be considered an old-era regiment but I believe their prime would have been considered 'new era'.