To achieve 89.164% ult cdr, you need:
Ally:
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Ornn (to have an upgraded Mythic item)
Runes:
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Ultimate Hunter (25% Ultimate CDR)
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Transcendence (10 Ability Haste)
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Ability haste rune shard (8 Ability Haste)
Items:
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Draktharr's Shadowcarver or Liandry's Lament or Starcaster Starcaster (25 Ability Haste + 25 from Mythic Bonus)
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Vigilant Wardstone (40 Ability Haste)
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Cosmic Drive (30 Ability Haste)
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Navori Quickblades (30 Ability Haste)
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Black Cleaver (25 Ability Haste)
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a 20 Ability Haste Item (e.g. Zeke's Convergence)
Buffs:
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Blue Buff (10 Ability Haste)
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4 Cloud Drakes (4*10% Ultimate CDR)
This gives a total of 10+8+(25+25)+40+30+30+25+20+10 = 223 Ability haste. Even though the ingame tool tip says that Cloud Drakes give 10 Ultimate Ability Haste, they actually give 10% Ultimate CDR (according to wiki and my tests). Also, the CDR from Ultimate Hunter and from the Cloud Drakes stacks additively (according to wiki and my tests).
So, in total, our cooldown is reduced by 1 - (100/(100+223))*(1-(0.25+0.4)) = 0.89164... = 89.164%.
To show that this is indeed correct, I tested it in practice tool. Unfortunately, I have 5 Ability Haste less, because I can't have an Ornn on my team. This means, that I only have 88.99...% CDR. I tested it with Morgana, who has 100s cooldown on level 16 ultimate. In this screenshot, you can see that she has 11.01s cooldown, which confirms that her cooldown is reduced by 88.99%. Proof
URF only gives you 80% cdr (but on all abilities). So Morgana's R would have a cooldown of 20s.
To show how absurd 89.164% cdr is, here are a few examples for level 16 ultimates:
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Lux: 4.33 seconds
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Karthus (together with Shen the longest cooldown at level 16): 17.34 seconds
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Nasus: 13s (Because his ult lasts 15s, he could have permanent ult. Unfortunately, proabily in patch 10.22, they removed the interaction where he could gain basically unlimited armor and mr https://youtu.be/K1oH3HSYv_8 )
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Teemo: 2.17s recharge time (Because a shroom lasts 300s, he could have 138 shrooms on the map; in URF the maximum amount of shrooms is limited to 20 afaik)
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Tryndamere: 9.75s (Because his ult last 5 seconds, he's basically unkillable half of the time)
Edit: Mixed up dots and commas as decimal seperators. I should have used a dot in the title.
