Rounded Defender (3.5e Optimized Character Build)
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If you need a tank, this guy is here. A maximum 46/45/47 saves and 108 AC, average 355 HP and a whole bunch of abilities, this build allows you to enter battle and leave it untouched.
Not entirely sure, basically looked things up on the web. [Editor's Note: Everything at a glance appears to be from core rulebooks. Please edit appropriately if the reader finds anything out of place.]
 Game Rule Components
 Spells, Powers, Soulmelds, Stances, etc...
You probably want a Spiked Chain for reach, wear no armor and shield.
Optional: Cloak of Resistance +5, Periapt of Wisdom, Robe of the Archmagi, Stone of Good Luck, Belt of Endurance, Crystal Mask of Mind Armor, Bracers of armor +8, +5 Amulet of Natural Armor, Monk's Belt, +5 Defending Weapons, +5 Ring of Deflection, +6 Gloves of Dexterity, +5 Tomes of Int, Wis, Dex and Con, Pale Green Ioun Stone.
Starting Ability Scores (Before Racial Adjustments): You'll want high (16) dex, int, wis and con. Try to get the highest overall bonus in those four.
Race (Templates): Dwarf
Starting Racial Traits: +2 Con, -2 Cha, 20 BLS, no speed penalty
I skipped this part because it would take forever, saves are explained, and it doesn't add too much value. Progression is totally up to you. You get Evasion, Uncanny Dodge, Deflect Arrows, Improved Uncanny Dodge (Deny 18), Combat Reflexes, Improved Grapple, Dodge, Mobility, Weapon Finesse, Defensive Stance 5/day, Mobile Defense, DR 6/-, Trap Sense +3, Trapfinding, Sneak Attack +2d6, Endurance, Toughness, no matter what, costing 5 feats. Good Feats: Improved Natural Armor, Combat Expertise, Superior Expertise, Iron Will, Lightning Reflexes,
Epic Feats: Bulwark of Defense, Damage Reduction, Improved Combat Reflexes, Epic Fortitude, Will and Reflex, Fast Healing.
You have 2 feats and 4 epic feats after class feats.
 Other Components
<- anything not covered by the previous sections ->
So, you want high saves and AC. At 4th level Sorcerer with highish cha you can have 3 2nd level spells, which you will use to buff Dex, Wis, Int and Con, Shield for 4 AC, and Heroism (from high Int) for another 2 AC and saves, and Reduce Person adds another 1 AC. 2 levels of Monk grants Evasion and AC from Wis, so with a buffed wis score you have +6 AC. 4 Levels of Rogue brings you to BAB +6, so you can go to Duelist, then Dwarven Defender. Dwarven Defender 10 and Defensive Stance gives +8 AC, DR 6/-, and 5 Defensive Stance per day. Duelist 9 gives you 18 AC if you got the items to boost int to +9. Dragon Disciple grants another AC and decent save boosts. The total saves are 18Fort/17Ref/20Wil. You can get them higher with equips and buffs pretty easily, resulting in max 46/45/47. Your AC is 40+Int Mod+Wis Mod+Dex Mod+equips, and all mods are buffed 2. You can get to 124 AC without too much trouble. You average 355 HP.
 Munchkin-Size Me
If you've got a caster, you can use walls of force or stone or your surroundings to place this guy in a bottleneck. Then he basically sits there and takes a nap while the enemies sit there and take ranged damage.
Not sure how MORE defense can be achieved other than more levels, though more levels of Duelist (3 epic feats, precise strike +3d6) or Dwarven Defender (10 Defensive Stance per day, Damage Reduction 12/-, Trapsense + another 3, 2 epic feats) or Sorcerer levels (More spells = more buffs, longer and more effective).
This build only gets really crazy defenses now and then. However, it gets excellent defenses the rest of the time. Magic Missile and other things that always deal damage kill this guy slowly, though with a Cloak of the Archmagi he gets SR 18.
 DM Counters
Use something akin to the Magic Missile Stormer, send a caster with no save spells, swarm him with x4 crit weapon people and kill him with nat 20s, send something that he can't block to get his allies, so he has no support. You could even grapple him.