As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
Here's a FAQ that includes the damage calculation stuff from the official Forum.
Just a few pointers, before you dive into it:
- There are three types of damage: Physical, Magic, and True. Physical and Magic are pretty clear, some abilities can deal true damage.
- All damage types are treated separately.
- The calculation is the same for all champions, but depends on the hitting champ and the one who was hit. However, each champion has different base values for armor/MR/damage/etc and different increments on level ups.
- The actual damage resulting from Phys/Mag damage depends on multiple things:
- the damage you currently dish out (what you see in your character info). This is made up as a simple sum of your base damage and any flat bonuses from items.
- your armor/MR reduction rate.
- your armor/magic penetration rate. These come in absolute values and percentages.
- your enemy's armor or magic resistance.
If you haven't run away yet, then check out this guide for mathcrafting LoL.
Best Answer
There isn't really a "best" marksman to farm with. Almost all ranged AD champions have the advantage of getting easy last hits due to the fact that they build AD and have basically unlimited resources due to the fact that they don't spend any mana on their "main attack", however there are some that have certain advantages.
Since there are usually two people (Marksman and support) on bot there are certain situations where different champions excel at farming while others are struggling. Therefore I will look at two main aspects of farming: The ability to farm minions and the safety while doing so.
In general there are a few things that contribute to the ability of a marksman to farm well. These are:
Attack speed buffs
These help in pretty much every situation. It just makes it plain easier to last hit and leaves more room for error since you might be able to add in a second hit if you screwed up your last.
Notable examples are Jinx, Varus and Ezreal
High range
High range makes it extremely safe to last hit and it allows for easy switching between harrassing and farming.
Notable examples are Jinx, Caitlyn and Ashe
Area of effect spells/Attacks
This makes waveclearing a lot easier and especially midgame you can kill multiple minions with a single use of an AoE spell.
Notable examples are Jinx, Sivir and Varus
Auto attack resets/Spammable abilities
Auto attack resets or spammable abilities are similar to an increased attack speed. They allow for more errors and can be used to quickly lasthit something.
Notable examples are Ezreal, Sivir and Vayne
Gapclosers/Defensive Spells
Last but not least we have gapclosers. This is again something that doesn't directly affect farming, however if you have the extra safety of a gapcloser you can usually go in for more risky creeps without getting punished too hard by the enemies.
Notable examples are Ezreal, Sivir and Kalista
Based on the factors above it's possible to narrow down the "best" farmers. Obviously these champions will perform differently based on mastery of the champion and the situation they're in but it's pretty safe to say that the following champions are extremely strong, safe or both while farming on bottom.
Jinx
Probably one of the strongest. She has AOE, high range, low mana cost, attack speed steroids and minor defensive spells. Her only real disatvantage is the fact that she doesn't have any gapclosers.
Sivir
Sivir has extremely strong waveclear while also staying relatively safe with her spellshield. She also has an auto attack reset and an attack speed buff past lvl 6. Her disadvantage is her low range and lack of gapclosers.
Ezreal
For people who are familiar with lasthitting as an ADC this will probably be the safest of all. You have a free flash every 6-10 seconds and a spammable Q plus an attack speed passive. The only disadvantage there is is the high skill cap of the champion.
Caitlyn
She's kind of like a mix between Ezreal and Jinx. While she doesn't have the mobility of ezreal or the damage of Jinx she has a mix of both that make her safe and strong while farming.
As I said. There is no real best champion but most mentioned above will outperform players on the same skill level that aren't playing those champs.