One of the best prestige classes for boosting armor class, and certainly the best for doing so while wearing heavy armor, is the Dwarven Defender.
Of the other core options, only the fighter, paladin, and cleric do not suffer some penalization of a core ability by wearing heavy armor. Druids are a special case, and I have a lot more to say about them in part two of this screed. Of the core prestige classes, only the Defender boosts armor class with no penalty to class abilities while wearing heavy armor.
Mention should be made of the Horizon Walker, however. The PrC's Desert Terrain Mastery offers immunity to fatigue, therefore letting an armored character sleep in the heaviest of armors with no penalty. The PrC requires Endurance, just as the Defender does. If you can swing the required ranks in Geography, you might want to dip into the HW.
But the Dwarven Defender really shines as an AC booster. It offers an escalating bonus of +1 to +4, and adds another +4 on top of that when the Defensive Stance class feature is activated.
An advantage to this path is that you do not need much in the way of stats. Push that Con up for hit points and longer duration Defensive Stances. Assuming you choose mithril full plate for your goal armor, then a Dexterity 14 gives you all the dexterity you'll ever need ... and if that's a problem, use Gloves of Dex to boost up to 14. If you go for admantine, Dex 12 will suffice. A decent Strength is always good for a tank, if only so that you can carry your armor around. If you go for cleric levels, a 14 Wisdom should be fine. Try to get a 13 Intelligence for Combat Reflexes.
And what classes? I would suggest Cleric4/Fighter2/HorizonWalker2, with Luck Domain (to allow hit die rerolls) and Knowledge Domain (for the ranks in Knowledge (geography) you need to become a Horizon Walker). But Ranger4/Fighter2/HW1 is an option (although you can't use your ranger combat style in heavy armor), and so is Cleric4/Fighter4, Fighter2/Paladin5, and others.
Skills are less important. 8 ranks in Knowledge (geography) if you want a couple levels of Horizon Walker. Even bought cross class, five ranks in Tumble is worth it.
The end result -- Let's look at a sample character created with these guidelines and the elite array (15, 14, 13, 12, 10, 8). We'll sample at levels 5, 10, and 15.
Level five (9,000 gp): Dwarf Cleric4/Fighter1 with 14 Dex. Feats: Dodge, Endurance, Combat Expertise. Gear: +1 full plate, +1 tower shield, +1 ring protection, +1 amulet nat armor. Armor Class: 27 (28 dodging).
Level ten (49,000 gp): Dwarf Cleric4/Fighter2/HW2/DD2 with 14 Dex. Feats: Dodge, Endurance, Toughness, Combat Expertise, Die Hard, etc. Gear: +3 mithril full plate, +3 tower shield, +2 ring protection, +2 amulet nat armor, +1 AC Ioun Stone. Armor Class: 36 (41 dodging with defensive stance).
Level fifteen (200,000 gp): Dwarf Cleric4/Fighter2/HW2/DD7 with 14 Dex. Feats: Dodge, Endurance, Toughness, Combat Expertise, Die Hard, etc. Gear: +5 mithril full plate, +5 tower shield, +5 ring protection, +5 amulet nat armor, +1 AC Ioun Stone. Armor Class: 48 (53 dodging with defensive stance).
Add in +5 for combat expertise and +3 for fighting defensively, and we're looking at AC61 at 15th level. Not too shabby.
This is part of an extended series on Survivability in Dungeons and Dragons.