Many summoners say that jax is underpowered and weak. I admit that i think hes underpower when i 1st played him too. but then, i suddently had the idea to arrange the skill arrangement and item which turned him very powerful. So im gonna tell you how and why in this build. i myself is not a pro in jax but i usually own him in most of my games

Greater Quitessence Of Insight x2
(This Is Just A Normal Great Thing For Champs Using AP)

Greater Quitessence Of Potency x1
(This Is For A Good Dmg On Early Game)

Greater Seal Of Evasion x9
(For More Dodge Chance)

Greater Glyph Of Insight x9
(Just A Normal Great Thing For AP Champs)

Greater Mark Of Insight x9
(Just A Normal Great Thing For AP Champs)

my core item is... all of them including the doran blade b'coz it can activate jax passive which gains hp for ad and lifesteal. but u can sell it late game for any other better items but make sure its AD for Health Points and base dmg. then guinso for great health addition and dmg + atk speed(stack).By now yor jax is quite tanky and you can kill people real fast with jus this skill sequence which i will tell you in the next section.

Early- You can harass your opponent with W,Q and take minimal dmg with the help of your lane partner. By the time you get your third skill(stun) you can go for a kill. When their health are low and and still farming minions, you can jus jump on him, stun(minions help),W and he should be dead. but if you have your ulti, it will be much more easier.

Mid/Late- Try to stick with your teammate and attend all team fights b,coz jax really shines in teamfight b'coz of his stun ability, damage and tankiness.. But never ever go in 1st like you're the boss.. you would eventually die for nothing and your team will rage at you..

Pros
-Great Burst Dmg
-Great Passive
-Tanky
-Great CC
-Often Aimed
-Great Farmer With Ulti

Cons
-Hard to master
-Not So Godly Early If Unfed
-Hate the casters!!!!!
-Vulnerable to CC which will break your ulti combo and make you start again with slow atks :3

-Needs Teamwork because jax is really team relying
-Farm well
-Dont go on 1v5.. 1v3 is okay. i usually win in 1v3 fight
-Try to enter teamfights little bit late so u can happily do your combo and finish their team in jus secs..

Have Fun And Happy Owning With Your Team!!