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.
Wow that is an incredible question to answer. Hasn't someone posted a lengthy review on runes for Udyr at like Mobafire? Because that where I would look. Rune combinations seem to show maximium effectiveness in clusters of 4. One type for each section. like health for seals. armour piercing for marks. Celerity for glyphs and health for quintencess. These are just examples. I would read a bunch of various reviews and make decisions from there. Items especially for Udyr will make a considerable difference and this will make your calculations more complex.
Best Answer
Both builds are definitly not viable for rankeds! Play them at own risk.
I'm not 100% sure if a i calculated the Health of Alistar corrently. I added all the flat bonus values (6007,76) and multiplied this with all percentages together (28%) and added the Base Health (2278): (6007,76 * 128 / 100) + 2278.
If there is a misstake, feel free to edit this answer but provide the calculation formula.
Alistar (without any skills involved)
the most health any champion can obtain is 9967,9328, being a level 18 Alistar with:
Base:
Runepage:
Masteries:
Items:
Sion (with skills involved)
unlimited health with Soul Furnace
Nunu (with skills involved)
the most health any champion can obtain is 13195.95, being a level 18 Nunu with:
All health runes and masteries:
9 Scaling Marks of Health (+87,48HP)
9 Scaling Glyphs of Health (+87,48HP)
9 Percent Seals of Health (+216HP)
3 Percent Quintessences of Health (+145,8HP)
3 points in Veteran's Scars mastery (+36HP)
1 point in Juggernaut mastery (+3%)
5 Warmog's Armor (+5000HP)
1 Spirit of the Ancient Golem (+200HP +25%)
An Elixir of Fortitude (+235HP)
Wild Growth cast by a Lulu with the maximum amount of ability power. (+???HP)
Consume cast on a Golem jungle creep (+10%)