The Fighter is one of the most diverse character in the game so it is recomended as a BEGINNER class. There are a great number of different stances and therfore you have the most to look forward to as you level the character up. The most useful and defining skill of a fighter is Provoke.

This brings me to my next facts. The fighter's usefulness and what stance to use in each setting.
For a more detailed list of what the abreviations stand for, click here.

PvEG - High
PvEP - Medium
PvEB - High
PvER - Very High
PvEF - Medium Low
PvPS - High
PvPL - Medium

Back-Guard > Sword and Shield: This is really too low of a level to be of much use as you progress
PvEG PvPS

Escrima > Dagger: I don't think I would ever use this stance with a fighter, only usefulness would be for attack speed with good DR.
PvPL

High-Guard > Sword and Shield lvl 12: Very good for tanking, but not useful when in lower levels.
PvER PvPS

Hack And Slash > Sword and Sword lvl 12: Well rounded stance that has AoE and DPS.
PvEG PvEB PvEF

Middle-Guard > Sabre lvl 12: Useless; you do need it, however, to unlock skills in Heaven or Hell.

Plow-Guard > Great Sword lvl 24: Great stance, personally one of my favorites. It works great for multiple situations, since it has an AoE skill as well as a self-buff skill. If nothing else be sure to get it to level 8 to unlock Roof Guard.
PvEG PvEB PvER PvEF PvPS

Blandir Cruz > Polearm lvl 24: Useful in a few situations, but not a late game skill.
PvEG PvEP

Freestyle Shot > Pistol lvl 24: I personally wouldn't use a fighter for a musketeer type stance but some find this works well with a fighter. I would at least get it to level 8 to unlock duality since I find that stance useful as a fighter.
PvEG PvEP

Roof-Guard > Great Sword lvl 36: Very useful, especially when paired with a high Flow Guard. Roof Guard is a high speed and damage version of Flow Guard, but you lose the defensive stance attributes of Flow Guard. Using Roof Guard for DPS and being able to switch quickly to Flow Guard if the main tank dies is one of the best strategies.
PvEG PvEB PvER PvPS

Double-Gun Shot> Pistol lvl 36: Unlike freestyle I do find advantages to using this skill with a fighter. With a combination of provoke and bloody overdrive as well as the higher DR that comes with being a fighter you can certainly do a great job stomping mobs. Its R.I.P. skills also allows you to take on area bosses too.
PvEG PvEP PvEB PvEF

Heaven and Hell > Sword and Pistol lvl 40: One of the more advanced stances to master. It's not so much of when to use the skills, its a matter of having the right equipment. DPS is the overall usefulness of this stance, but in order to raid sucessfully you need good DR since you get no defense bonuses with this stance.
PvEG PvEB PvER PvPS

Epee Guard > Rapier lvl 48: The more offensive of the two level 48 rapier stances. Attack speed and its crit rate make it useful in single target attacks, but this stance struggles when faced with multiple targets.
PvEG PvEB PvER PvPS PvPL

Sabre Guard(Short Guard) > Rapier lvl 48: The more defensive of the two level 48 rapier stances. The evasion makes this great for powerleveling but evasion isn't the greatest for raids. The skills are decent but nothing worth mentioning.
PvEP PvPS

The Defender > Shield and Sword lvl 100+: The ultimate for tanking, especially with raids. This stance allows for self buff as well as multiple skills that disable.
PvEB PvER PvPS PvPL

Stave Guard > Shield and Sword lvl 100+: One of my favorite stances, it's a well rounded stance that also looks cool. It's fast paced enough that you can grind with it. It's a good mix of DPS and DR, though you need to use the self buff. Definitely a good one for backing up a tank on a raid.
PvEG PvEB PvER PvEF PvPS PvPL

Sidewinder > Great Sword lvl 100+: A great final addition to the great sword stance. This stance captitalizes on AoE; even the standard attack can hit multiple targets at once. By completing the trifecta of the great sword stance you can easily switch between each stance depending on the situation without switching the weapon.
PvEG PvEP PvEF PvPL

Avant Garde > Rapier and Pistol lvl 100+: Though I personally haven't used it yet, from what I gather its emphasis is on DPS and single target destructio;: its one skill when maxed does 3000% attack as well as a disable.
PvEG PvEB PvER PvPS *this is subject to change since I don't use it*