Well, I would say Varangian Guard(and I think they are the best medieval infantry of all times) because it's in my fav empire and also because they are very powerful at attack and also at defence...
Also, in late Medieval times I would also prefer Swiss Armoured Pikemen( the Swiss are my second fav after the Byz).....

But I would also say the samurai(especially Kensai)....

I think BKB was not reffering to the mercenary catalans the Byzantines hired....