Biggest munchkin?
Hmm, that's a toughy, cause there are limits as to what even a gm will allow.
If you were going to push your limit, you could try convincing him to allow a gestalt character. Rules for gestalt are in the CA.
There are even more extreme munchkiness available there if you. If you understand the rules there you could pull something like my friend tried to do.
Warlock 10/ Incarnate 2, Binder 5, Saint 2, Hellfire Warlock 1 for his 10th lvl gestalt.
gestalts technically don't have an LA so you could pull it over on your gm, but our campaign treats them as a +3 LA
Ignoring gestalts, my personal favorite for munchkyness is warlocks.
Two possible builds, both completely legit.
My build: very high dex. Main weapon: glaive
Defense: Max dex out while getting no negatives on anything but str.
Combined with celestial armor to get 8 armor while being able to get another 8 ac from dex, and then ring of prot 5, amulet of NAC 5, gives you a whopping 36 AC. You can get still more if you work at it.
Attack: Eldritch blast always hit touch AC. Using the glaive and a hast will give you a full attack +1 for a 3/4 BAB. At will invisibility, flying. Use acid modifier to your glaive, lets if bypass SR and DR. Take the maximise spell like abiltiy feat to maximize it 3 times a day.
Other cool ability options like devour magic or summon swarms. Did I mention 10ft reach with the glaive? No line specifically stating you cant hit adjacent squares either, as glaive would.
Magic Items:
Many many possibilities. For example, you could create a pendant which as a free action, gives you true strike, If you pay double, it be made so that it won't actually take the armor slot that it's made for, so you could stack it with the amulet of NAC 5. For a 30% diiscount, you can have the item made for your specific alignment.
Belt of battle is awesome, especially for glaivelock. Since the glaive lasts till the beginning of your next turn, it gives you AoO, but also, th maximize will last through another full round attack you can crank out from this baby.
Finaly bit of munchkiness I just found out is Black Blade of Destruction. Too high for a lvl 11, but get some poor soul to craft this baby for you at max level when you have the opportunity. Stacks with hideous blow from warlock, allowing you to deal 40d6(+9d6 (+6d6 if you take hellfire warlock prestige) +12d6 from various items you can buy to pump your EB)
In the end that's my formula, but I've seen a warlock optimizer out there which claims to allow you to get 2000 dmg approx from a maximized hit while still getting your full attack. With haste, at 20th . . . .you can do the math, but I doubt many monsters can survive that,