Defensive Specialist.
Well, I'm sure other people have managed to get it higher, but my character is a defensive specialist... at 19th level, he can get his AC up into the 70's, has higher resistances, damage reduction, and quite a few immunities. For extra twinkiness, a level of monk will give you an extra few points of AC, while two levels of monk will give you evasion. Here's the build, assuming you're using monk levels:
Human PsiWar 18/monk 2. Starting Dex 16. Starting Wis 18, all stat boosts to Wis
Relevant Powers:
1st Level: Inertial Armor, Thicken Skin, Defensive Precognition, Biofeedback (Can be changed with Inertial Barrier, but Biofeedback has higher potential), Force Screen (Can be omitted if you can use an animated shield)
3rd Level: Concealing Amorpha, Greater, Empathic Feedback, Evade Burst (can be omitted if you took two levels of monk)
4th level: Energy Adaptation, Freedom of Movement, Metamorphosis (taken with Expanded Knowledge)
6th level: Dispelling Buffer (necessary with all the buffs you have up), Personal Mind Blank.
Items: Ring of Protection +5, Weapon of Parrying and Defending +5, Cloak of Resistance +5, Gloves of Dexterity +6, Periapt of Wisdom +6, Tome of Understanding +5 (used), Tome of Quickness in Action+5 (used)
Feats: Dodge, Psionic Dodge, Combat Expertise, Superior Expertise (Oriental Adventures), Psionic Weapon, Deep Impact, Expanded Knowledge, Psionic Meditation
Skills: Tumble (Max ranks (11 CC, 22 class) (Important if using OA tumble rules, otherwise only 5 ranks are necessary)
I haven't gone through all the monsters, but stone giant works fairly well as an alternate form. Now comes the adding:
1) Dexterity. You're not wearing any armor, so you get your full dex bonus... which, what with one thing and another, is +8. AC so far: 19, FF 10, Touch 19
2) Monk AC Bonus: Again, no armor, so you get to apply this. You Wis bonus is +12, so there's another 12 points. AC so far: 30, FF 22, Touch 30
3) Ring of Protection +5. AC so far: 35, FF 26, Touch 35
4) Inertial Armor. At 18th manifester level, you can augment this up to 17 power points, for a total armor bonus of +12. AC so far: 47, FF 38, Touch 35.
5) Thicken Skin. At 18th manifester level, you can augment up to 16 PP, for a total armor enhancement bonus of +6. AC so far: 53, FF 44, Touch 35.
6) Weapon of Parrying/Defending: Gives a +1 insight bonus to saves/AC, and you can use the +5 as an AC bonus. AC so far: 59, FF 45, Touch 41
7) Expertise/Superior Expertise: Gives up to a +14 Dodge bonus to AC. AC so far: 73, FF 45, Touch 55
8) Stone Giant: Your dex drops by 2 points, and you get a -1 size penalty. However, you get a +11 natural armor bonus. Takes a round to bring up. AC so far: 82, FF 55, Touch 53
9) Defensive Precognition, prepared: If you have prep time, you can pump all 16 points into upping the bonus, giving yourself a +6 Insight bonus (+5 net, since your Parrying already gives you a +1 insight bonus) to AC and saves. AC so far: 87, FF 54, Touch 58
10) Defensive Precognition, quick: If you don't have the time to spend an extra round buffing, you instead get a +4 bonus (+3 net). AC so far: 85, FF 54, Touch 56
11) Force Screen: Augmented to 17 pp, for a total shield bonus of +8. It takes a round to put up, however. AC so far: 93, FF 62, Touch 56 (note, however, that you usually won't get this bonus while FF)
12) Fighting Defensively: Depending on your DM, you may or may not be able to stack with Expertise, but there's nothing preventing it from stacking. Depending on if you're using OA, you'll get either a +3 or +4 dodge bonus. We'll assume +3 for this build. AC so far: 96, FF 65, Touch 59.
13) Dodge/Psionic Dodge: Against one opponent, you get a +2 dodge bonus to AC, or +1 vs. two opponents. AC so far: 98, FF 65, Touch 61
Additionally, throwing on a Monk's Belt will add another +1 to all AC's.
With your other powers, you gain DR 7/-, Energy Resistance (everything) 30, a Mind Blank effect, a Freedom of Movement effect, and total concealment (with one round of manifestation), and (fully buffed) saves of Fort +25 (+con), Ref +27, Will +31. You have 202 pp's, of which about half will be spent on your first buff of the day, which will last you 3 hours before you have to reapply most of it - Extend Spell might be a useful investment at this point. You're vulnerable to dispels, but thanks to Dispelling Buffer, they have to hit a DC 34 to dispel your stuff. With Deep Impact, you can still hit things even while going full-out defensive - unless you have to hit yourself. =b If anything, you're the guy who'll bring everyone back after they die a horrific death.
You are, however, a sitting duck for a beholder.
This isn't my actual character - he's chaotic, has been since level 1, so he can't go Monk. He sort of evolved into the role of defensive specialist after an unfortunate encounter with a thri-kreen that ginsued him with eight attacks in one round, at 7th level... since then, post-traumatic stress disorder has turned him into the defensive powerhouse he is now, with a routine AC in the 40's, and high AC around 70 or so. Had I been intending the obscenely min-maxed version I just presented above, he'd be really terrifying, and my DM probably would have thrown the three beholders that finally killed him (after 12 death-free levels) far earlier than he actually did.