League-of-legends – Single item that offers the most cooldown reduction

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.

Cooldown reduction (CDR) has a hard cap at 40%.

You can get 10% CDR from masteries alone (9/0/21), and 7.50% from flat cooldown glyphs or 10% from scaling CDR glyphs, a little more if you invest in CDR seals or marks (though they will provide a lesser amount than the glyphs.

If you're playing Summoner's Rift, the Crest of the Ancient Golem provides a buff giving scaling mana regen and a flat cooldown reduction bonus of 10%.

You can hit the cap with a 9/0/21 mastery page in conjunction with scaling CDR glyphs, a blue buff, and an Elixir of Brilliance (blue pot).

However, you cannot hit the cap using only masteries and runes. You can get up to 32.5%: 10% from a 9/0/21 mastery page, 1.8% from flat CDR marks, 3.2% from flat CDR seals, 10% from scaling CDR glyphs, and 7.5% from scaling CDR quintessences.