Lastly but not least is Mr Fantastic
Reed is the most difficult of the four to get right. That's why he's last, he took the most time to think about.
Movement
Live-action films have yet to get someone with super elasticity right, so getting Reed right would be a first in Hollywood. However there is one genre that has gotten close and that's horror.
Reed's body moves and contorts in ways that if we saw it in real life would be very disturbing and that's the key to making it look believable. It should look kind of gross and disturbing and applying techniques commonly used in horror films may be what's needed.
I won't post any examples because... you know.... horror. So I'll post this instead to show that when you think about it Reed is basically body horror.
Appearance
Combining prosthetics with CGI is the way to go for Reed.
It's difficult to go into detail here because it's all about getting the special effects to look right which is all about how the SFX and VFX teams handle it. They have their work cut out for them when it comes to Reed.
How he fights
Reed is the one who should be the last one to want to fight. But when he does it's mainly using a gadget he's created or restraining opponents with his body and grappling them.
When it comes to him having to be more aggressive they should a few queues from one of the most popular elastic heroes currently. Luffy from the anime called One Piece.
Luffy's attacks take start-up because he uses his elasticity to it's potential. Instead of just punching someone he stretches his arm back and then releases it to increase the force of his blows. He does that with most of his attacks. That's how Reed should fight, maximizing the use of his rubber physiology.
So if Reed were to kick someone his leg would extend back a bit and the release forward into his target like when you pull a rubberband.
In terms of powers Reed is the most difficult of the four to get right visually in live action films. It will take a combinations of techniques and maybe some innovation to get his stretching to look good. But if done right you can have some really cool scenes.