Plastic Man:
>Stretchs his body in a molecular level, so he can take any form he wants, even giant forms or sharp forms
>Resists fire, but too much heat can melt him down
>Stretchs inside a person's body through any orifice, being able to cause death by organs damage or asphixia
>Resists phisic damage due to his gum body in an extreme level
>Immortal, somehow
>Regenerates from dismemberment, but this could cause psicological traumas
>Had enough will power to regenerate himself from deintegration, but it took him hundreds of years and he needed help from the Justice League to gather the particles of his body

Luffy:
>Resists phisic attacks with Armament Haki, even sharp attacks
>Feels the enemy's precense and movements with Observation Haki. And now he can slightly predict future
>Moves extremely fast with Gear Second
>Grows his limbs with Gear Third
>Moves even faster with Gear Fourth, and becomes strong enough to break an entire city in two with one hit
>With Red Hawk, he can make a fire hot enough to burn under water
>Water is his weakness, if he is submerged in a big amount of water he loses
>Turns an elemental body into a normal human body hitting with Armament Haki, but it is not known if it would work with Plas
>Literally instant knocks out the enemy with Emperor Haki, if the enemy's will power is weaker
>Survived to being crushed and locked inside a box smaller than an apple, without using Armament Haki

Ok am I forgetting some ability?

So who would win in a fight? Not necessary to be a death fight because it wouldn't make sense since Plas is immortal