What are the best/coolest martial multiclasses in your opinion?

[Windrammer]

I find Rogue 3/Fighter x to be a charming one.

[Tvtyrant]

Rogue 19/Barbarian 1 (barbarian would be second level.) Nets you much better AC and resistance to none-magic weapons. Also allows you to play the Stronk Rogue, with strength instead of dex and using a rapier to get auto-advantage on your first hit a turn.

[Windrammer]

Rogue 19/Barbarian 1 (barbarian would be second level.) Nets you much better AC and resistance to none-magic weapons. Also allows you to play the Stronk Rogue, with strength instead of dex and using a rapier to get auto-advantage on your first hit a turn.

Can you explain the auto-advantage bit?

[Tvtyrant]

Can you explain the auto-advantage bit?

Barbarians get Reckless Attack, which grants advantage to their attacks when using strength in return for granting advantage to their enemies. The Rogue can sneak attack as long as it is using a finesse weapon, but it doesn't specify you must use your dex. A strength Rogue with 2 levels of Barbarian (sorry I forgot it was level 2) can apply advantage and so sneak attack by running up and stabbing as hard as they can.

[Madfellow]

Monk 2 / Barbarian X

Basically turns you into this:

http://www.artofmtg.com/wp-content/uploads/2014/07/Khans-of-Tarkir-Art.jpg

[The Dolman]

Monk 12/Fighter 4/Rogue 4 is hilarious as a grappler (http://community.wizards.com/forum/player-help/threads/4142801). (Specifically the 10th entry in the guide)

[Rfkannen]

Warlock/ shadow monk

Probably 2/18 but any would do.

stat wise you get the whole, seeing in darkness, makeing darkness, than jumping into said darkness.

Flavor wise you get a monk(the religiusy kind) that worships a false god. Or maybe a heratic from a curch. All that kind of stuff.

[Phion]

16 barbarian (bear totem path to tank for days and become silly strong becoming a hercules like character +all the other cool stuff)/1 fighter (reroll 2handed weapon damage + heal as bonus action)/ 3 rogue (sneak attack is nice with shortswords if you need to pull out a side weapon if maul is gone, skill expertise in stealth (+14) and athletics (+17) so you can basically grapple or knock prone creatures (advantage with rage as it is strength related) with ease gaining advantage on attacks against them (can do this as one of multiattacks), cunning action to disengage to control battlefield (sentinel feat) dash 80 feet (barbarian unarmored movement) and hide (+14 stealth remember you are no slouch here, use longbow as well from a distance) and assassin so both your attacks deal crit damage on a hit (combine barbarian crit feature for extra dam and fighter feature to reroll 1's+2's) so 6d6+9 (not including magic items dam) x2 if both attacks hit.

Get the toughness feat and become a true warrior like Phion Grliv protecter of the mercenary group Clan Muffin Packin.