Many People Ask Me "Whats The Best Armor For Fallout 3 With DLC"? In My Opinion I Would Say 'Winterized T-51b Power Armor' From 'Operation: Anchorage' DLC. You Get The T-51b Armor From Completing All The Quests In Alaska. After You Beat Them All They Take You Out Of The Simulation And Grant You Access To Their Armory. Which Inside Is The 'Winterized T-51b Armor' Also Other Unique Weapons From Alaska.
Reason For This Armor: Because Its Condition Doesn't Go Down, And It's Body Armor Has 45 DR, And Has +25 Rad Res. The Power Helmet Has 10 DR And CHR +1, +8 Rad Res. Which Is Extremely Good.
I Use It 24/7 And I Have 510 Hp And I Hardly EVER Die. Usually The Only Things That Can Kill Me Are 'Deathclaws' But Yeah. Best Armor Is Winterized T-51b Power Armor.

Now For The People Without DLC:
You're Best Armor Would Be T-51b Power Armor As Well But Its Just Not Winterized.
Location: Inside Fort Constantine In A Room With A Dead Raider Infront Of A Door. You Must Search Them And Get The Key To Get Access. Then Unlock The Door And Theres A Blue Force Field Type Thing. Then You Must Hack The Terminal And Disable The Force Field. Simple As That.
T-51b Power Armor: Body Armor Is 50 DR & Has +25 Rad Res. The Power Helmet Has 10 DR & CHR +1, +8 Rad Res.
Well There You Go People! Repy Any Requests Or Message Me And I'll Answer Any Questions.