I'm also heading a project on a D&D optimization board to make the most invincible fighter possible without using infinite loops, with the goal of beating any wizardly counter. I'm winning.
This I have to see. I promise, I won't tell my own D&D optimization board, or even make inane comments on yours about it!
I'll be sure to let you know when it's finished. At the moment I haven't even focused on damage output yet. Right now, without using any magic at all, it is immune to...
Fire
Frost
Shock
Negative Energy
Positive Energy
Necromantic Effects
Acid
Sonic
Prismatic Effects(like prismatic wall, etc.)
Poison
Nausea
Stunning
Blindness
Aging
Nonlethal Damage
Critical Hits
Falling Damage
Mind-Effecting Effects
Ability Damage
Ability Drain
All Spells Under 5th Level
...And a couple others that I'm probably forgetting right now. It's also able to replace any save with a concentration check that it can always take 20 on, though it doesn't guarantee an autosuccess. It's still far easier to boost Concentration than three separate saves. I'm trying to find as many things as I can to link to this skill.
I'll probably add in some magic-based stuff, but right now it is all about having protection that will function whether there's antimagic about, my equipment gets disjoined, or other such silly things. Also, it can fly without magic, so that's neato.