Looking around on the GuideScroll Warframe guides I have found the following information:
GRINEER: Takes bonus damage from Armor Piercing.
CORPUS: Takes bonus damage and gets staggered by Electricity.
INFESTED: Takes bonus damage and some gets staggered by Fire, some are immune to Electricity.
Freeze damage dose 200% damage to shields where all other damge type do 100% damage.
Most enemies will recive a bonus resist based on there level that is subtracted from your damage.
Damage is listed as Normal hit/Weakpoint hit, for most that is Body/Head.
GRINEER
Everything is armored except for melee Grineer.
Normals include trooper, Lancer, etc.
Heavy include Bombadier and Gunner.
Shield Grineer takes damage as Normals, but recives 2 levels less resistance.
Commander-normal is the same as Heavy but immune to electrical/fire damage, AP/freeze not tested.
Normal Armor Piercing Fire Freeze Electrical
Sawmen 100%/300% 100%/200% 100%/200% 100%/200% 100%/200%
Normals 33%/100% 150%/300% 37.5%/75% 16.5%/33% 20%/40%
Heavy 66%/200% 100%/200% 50%/100%+ 50%/100% 50%/100%
CORPUS
Crewmen are armored.
They are Staggered by Electricity.
Moa's weak point is there midsection/gun.
Normal Armor Piercing Fire Freeze Electrical
Crewmen 81%/0% 100%/2000% 33%/100% 75%/200% 160%/400%
Moa 100%/200% 50%/100% 50%/100% 50%/100% 200%/400%
Osprey 100% 100% 100% 100% 200%
Cameras 100% 100% 100% 100% 200%
INFESTED
Normals include Chargers, Leapers, Runners, etc.
Ancient heads/bodies and Crawler bodies are armored.
Normals and Ancients are Staggered by Fire.
Crawlers have 3 areas of damage Body/Legs and Arms/Head.
Ancients have 3 areas of damage Body/Head/Lower legs and Arms.
Normal Armor Piercing Fire Freeze Electrical
Normals 100%/200% 50%/100% 200%/400% 100%/200% 50%/100%
Nauseous Crawler 66%/100%/200% 50%/50%/100% 125%/200%/400% 50%/100%/200% 50%/50%/100%
Crawler 50%/100%/200% 50%/50%/100% 100%/200%/400% 50%/100%/200% 50%/50%/100%
Ancients 25%/75%/100% 100%/200%/100% 37.5%/75%/200% 25%/50%/100% 0%
I don't recall how many Warframe and weapon slots you start with (I've bought several expansions for each), but here's how you can check how many you have of each:
edited for update 10 UI overhaul
In the upper right, pick the Arsenal button. Then pick the Inventory button (right above mods). By default you should now be looking at your Warframes; it will list each Warframe you have, its rank, how much it sells for (almost always 10k credits), and also any empty Warframe slots you have. If you look in the upper left of this display, the next button to the right of the Warframe selection is weapons. If you pick that all of your weapons will be listed, along with their ranks & sell values. As with Warframes, if you scroll to the end of this list it will tell you how many empty slots you have.
I think you start with 2 Warframe slots and 6(?) weapon slots.
Best Answer
Well, that's gonna be depending on a few things. You need to remove armor from entire rooms or just the next enemies you're going to hit? An efficient Ash with Seeking Shuriken and some power strength will strip 100% of the armor of something around 2-4 enemies per cast (he throws 2 shuriken and they have punchthrough, accounting for multiple hits). A Mag with the Fracturing crush and some power build can strip 90%+ armor in two casts with decent power strength. The effects are temporary but if you use Polarize once, it should strip the remaining bit, which will also make it permanent. This works even for heavy units at lvl 150, so more than enough for the vast majority of missions.
At the moment, Mag, Oberon, Banshee, Ash, Trinity, Hydroid can remove enemy armor entirely while Frost, Nekros and Vauban can only reduce it. Out of these, we have Oberon, Hydroid, Banshee, Nekros and Frost which can do it on a wide aoe (remove Nekros and Frost if you want complete armor removal). But that may not be enough. If you want Warframes which can remove all armor with one cast, you're down to Banshee for large aoe, or Ash and Trinity.
It's hard to state which is best from this, since each has their own domain.
Oberon requires enemies to be standing on his hollowed ground then using Reckoning 2 or 3 times to completely strip their armor, but it can affect a large area and is permanent so I might rate him as best overall.
Banshee would be best for quick aoe strip. With the Sonic Fracture and some power strength, she can fully strip armor (for 8s, affected by duration) in a (large, depending on power range) cone in front of her. One cast at 145 power strength will strip 100% armor and it's considered a one handed action so you can do it during parkour or while reloading. No setup or multiple casts required and it's a first ability so cheap and easy to spam.
On third I'd place Ash. My reason is, unless enemies levels are in the thousands, you don't need to completely strip armor from everything around you. Ash allows you to focus on whichever target is most threatening and quickly kill them while his Teleport and Bladestorm allows for attacks that ignore armor.
The others I'd rate below because they either take longer (Mag needs to use Crush more the higher the enemy levels, followed by Polarize to remove the last percentage to make the armor strip permanent) or don't offer full control on which enemies gets stripped (Trinity's Links, Hydroid's Corroding Barrage)