If you use a Wild Shape Ranger to gain entry into Master of Many Forms you have a lot more types of armor to choose from than the Druid. Going with the Wild special ability is more or less a given at higher levels (with a Wilding Clasp at lower levels) but what kind of armor should it be put on? Sure there are quite a few types of light and medium armor in the core books, but should you get one of those or should you look at something from another source? Books allowed are the 3.5e books and 3e books that were not updated (such as the Arms and Equipment Guide).
A chain shirt is the right choice for almost every character, sadly enough. It provides a fairly big wodge of AC, minimal armor check penalty, and has a nice, big maximum Dexterity limit.
In theory, a lot of characters could swap this for mithral full-plate if they have medium armor proficiency, but in practice it’s usually not worth it; armored AC just isn’t very valuable, and that’s an expensive armor. But the point is moot anyway, since rangers lose class features if they wear armor heavier than light.
Anyway, in general, the only reasons to use another armor are if you
- have extremely high Dexterity, or
- have quite-low Dexterity and heavy-armor proficiency and are not an arcanist, or
- have class-specific requirements to use something else.
For those with very-high Dexterity (expect to reach 24 or higher), a chain shirt is limiting. In that case,
- Spidersilk, +3 AC, +8 max Dex (Underdark)
- Nightscale, +2 AC, +10 max Dex (Underdark)
- Gnome twist cloth, +1 AC, no max Dex (Races of Stone)
For those with quite-low Dexterity (maybe 15, but probably not even then; mostly we mean 13 and less) and heavy armor proficiency, full-plate may be better. You get +4 armored AC relative to the chain shirt, and since your Dexterity isn’t that high anyway, you don’t lose AC. But armored AC is not particularly valuable, and that −5 (after masterwork) armor check penalty is prohibitive for a lot of people. Like the high-Dexterity folks, there are additional armors in supplements, such as Races of Stone’s mountain plate, but the armor check penalty problem becomes even more severe, plus many of these require Exotic Armor Proficiency, which is just not worth it.
Almost no one should use any armor between a chain shirt and full-plate. Breastplates have double the cost, double the penalty, reduced max Dex, and heavier weight, all for a measly +1 armored AC; not worth it at all.
Obviously, if you have a class feature that says you must wear some other armor, or you can’t wear certain armor, you have to pay attention to those and they can change any of the above. The monk is the obvious example. But as noted above, in your case, the ranger itself does have class-based armor requirements – specifically, to wear light armor. A number of their features don’t function in medium or heavy armor.
If you have high, but not very high Dexterity, in the 20-23 range, you just have your chain shirt made out of mithral. It’s relatively cheap, and has very few downsides. Mithral can also dramatically improve full-plate as an option, improving that −5 to a far more manageable −2 and allowing your Dexterity to get up to 17 without the armor limiting you. Whether this is actually worthwhile is debatable; armored AC really isn’t worth much, and that’s a lot of gold for +4 armored AC over a simple chain shirt.
Also, avoid wild
The wild armor property is very expensive, as a +3-equivalent, and it only keeps the armor bonus of your armor, none of its special properties. The special properties are about 85% of the point of having armor in the first place; this is not a good choice. (If you do go with wild though, you might as well get the biggest, heaviest, highest-AC armor you can find, as recommended by Scrollreader.)
Wildling clasps, on the other hand, limit you to forms that could reasonably wear your armor. As a master of many forms, that’s a pretty dire limitation. No good.
But the beastskin special armor ability allows you to continue to wear your actual armor, complete with its other special armor properties, on any form you like. It’s priced as a +2-equivalent and requires an extra use of Wild Shape every time you want to use it, which is brutal, but by keeping the wildling clasp you can limit how often you need to do that to only those forms that cannot wear your regular armor, conserving Wild Shape uses. This seems to be the best solution.
Wild armor does not have an Armor Check Penalty or a Max Dex when you are in Wild Shape. Normally, when you Wild Shape any equipment worn or carried melds with your new form and becomes nonfunctional. All the Wild armor property does is say that you continue to benefit from your armor (and enhancement) bonuses to AC when this happens.
If you don't mind a bit of cheese (or if your game is relatively high powered) you should wear Mechanus Plate Wild armor as a Wildshape ranger. While you're in Wild Shape, it has no armor check penalty or max dex.
Beware getting caught in human form, however, as the penalties will be crippling. If you're worried about antimagic or sleeping, make it from Mithril so you have proficiency.
If you find this a bit too rules lawyery, stick with something light with a max dex that matches a few favorite forms and make it Beastskin or stick a wilding clasp on it. A mithril chain shirt is usually an excellent choice for everything but the highest dex forms.