1) Luffy vs Lucci because the style they used. Just pure brawling hand to hand combat. Martial arts and street brawling.
Luffy and Lucci were close with luffy being slighty stronger in Gear 2.
2) Luffy vs the Admirals. Why you ask? The reason is because Luffy would learn new techniques from them and they know rokushiki.
Luffy fighting Yonko's is useless as they are stupid when it comes to combat with the exception of Shanks and Whitebeard.
They are the most balanced fighters, something Luffy is and is becoming more balanced as time goes on.
It would be like the lucci fight,