What does everyone think is the best multi class for a fighter/ favorite multi class for a fighter?

You will get a million different opinions because the Fighter class is just so darn versatile.  However I will start things with a fun build I am toying with - the Hex Knight.  Start off with a level of Fighter then multiclass into Hexblade Warlock for 1 level to get the Hex Warrior ability and Eldritch Blast cantrip.  Invest the next 6 or 7 levels Fighter picking the Eldritch Knight subclass and maxing CHA with the ASIs you pick up along the way.  Then level up the Warlock side to 3 or 5 to complete the build.

It is a fairly SAD build requiring only 14 DEX besides CHA.  You can be a menace in either melee (Booming/Greenflame Blade with a longsword) or ranged (Eldritch Blast and a hand crossbow/longbow) and you are also capable of filling the face role for the party.

How good is rogue with barbarian I’ve heard it’s ok what your opinion?

Quote from Moon137 >>

How good is rogue with barbarian I’ve heard it’s ok what your opinion?

 There do work fairly well together as the Barb side gives guaranteed Sneak Attacks via their Reckless Attack feature.  You can go anywhere from 2 to 6 levels of Barb before going Rogue the rest of the way.  The minimal of 2 gets you Reckless Attack, 3 gives you a Barb subclass, 4 unlocks a ASI, 5 gives Extra Attack while 6 can unlock a useful Barb feature while also increases your daily Rages by one.

However the drawback of more Barb levels means you build up your Sneak Attacks more slowly and giving your foes advantage on their attacks on you.  Also you will most likely be using medium armor which sadly gives you disadvantage on Stealth checks.

Quote from ThelonelyMagi >>

Quote from Moon137 >>

How good is rogue with barbarian I’ve heard it’s ok what your opinion?

 There do work fairly well together as the Barb side gives guaranteed Sneak Attacks via their Reckless Attack feature.  You can go anywhere from 2 to 6 levels of Barb before going Rogue the rest of the way.  The minimal of 2 gets you Reckless Attack, 3 gives you a Barb subclass, 4 unlocks a ASI, 5 gives Extra Attack while 6 can unlock a useful Barb feature while also increases your daily Rages by one.

However the drawback of more Barb levels means you build up your Sneak Attacks more slowly and giving your foes advantage on their attacks on you.  Also you will most likely be using medium armor which sadly gives you disadvantage on Stealth checks.

 In addition, rogues use finesse weapons, and barb features like rage and reckless, require you to use str for the attack. So you’re basically locked in to using a rapier with str to hit, which is viable and RAW, but kind of skirts around whether it’s RAI. But you definitely can’t sneak attack with, say, a greataxe or a maul.

And to the OP: what are you trying to do with this multiclass? And do you consider fighter the main class or a dip? Most every class can benefit from 2 levels of fighter, for example. Also, what fighter subclass are you considering? Some work better with some multi classes than others.

Quote from Moon137 >>

What does everyone think is the best multi class for a fighter/ favorite multi class for a fighter?

WAY too broad a question. It all depends on what you're trying to achieve by multiclassing, either mechanically or storywise. 

Totam Barbarian(Bear)with PAM, Sentinal to 6th lvl, Fighter Cavelar till lvl 11 for a third attack andthen Barbarian till 9. The Cavelar is awesome for defending your allies and locking down battlefrield. Goes super well with aBear Barbarian reckless attack, resistant to all damage types except psychic.

Cavelar 

Unwavering Mark

Starting at 3rd level, you can menace your foes, foiling their attacks and punishing them for harming others. When you hit a creature with a melee weapon attack, you can mark the creature until the end of your next turn. This effect ends early if you are incapacitated or you die, or if someone else marks the creature.

While it is within 5 feet of you, a creature marked by you has disadvantage on any attack roll that doesn’t target you.

In addition, if a creature marked by you deals damage to anyone other than you, you can make a special melee weapon attack against the marked creature as a bonus action on your next turn. You have advantage on the attack roll, and if it hits, the attack’s weapon deals extra damage to the target equal to half your fighter level.

Regardless of the number of creatures you mark, you can make this special attack a number of times equal to your Strength modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.

Warding Maneuver

At 7th level, you learn to fend off strikes directed at you, your mount, or other creatures nearby. If you or a creature you can see within 5 feet of you is hit by an attack, you can roll 1d8 as a reaction if you’re wielding a melee weapon or a shield. Roll the die, and add the number rolled to the target’s AC against that attack. If the attack still hits, the target has resistance against the attack’s damage.

You can use this feature a number of times equal to your Constitution modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.

Hold the Line

At 10th level, you become a master of locking down your enemies. Creatures provoke an opportunity attack from you when they move 5 feet or more while within your reach, and if you hit a creature with an opportunity attack, the target’s speed is reduced to 0 until the end of the current turn.