I'm not going to List every Fight he's been in so if you notice I've skipped one or two don't worry about it.
Luffy vs Arlong (Fishman)
Luffy vs Crocodile (Logia)
Luffy vs Enel (Logia)
Luffy vs Lucci (Zoan)
Luffy vs Gekko Moriah (Paramecia)
Luffy vs Boa Sisters (Zoan)
Luffy vs Magellan (Paramecia)
Luffy vs Hordy Jones (Fishman)
Luffy vs Caesar Clown (Logia)
Luffy vs Doflamingo (Paramecia)
Im sure we can all agree the toughest battles Luffy has been in are usually him vs a Paramecia user and Lucci, Zoan user. The only person i can think of to put up a great fight on the same level as Lucci or Doflamingo as a Logia user is Akainu.
By "great fight on the same level..." I'm referring to the hype, length of the fight, emotions, sheer willpower pushing past the bodies limits and all that good stuff