Quick Answer
Zuri's best ability route depends on the problem. The Cruel is the current boss-improvement route after Patch 0.5.3 buffs. The Serpent's Kiss is the Poison and sustain route. The Bladedancer is the Rush/Stall self-healing route.
The Cruel
Pick The Cruel when you need Zuri to help solve bosses. The material connects it with Petrification, Magic scaling, Poison from Stone Skin, Stun, and direct Patch 0.5.3 buffs. It is the clearest “make Zuri better into bosses” answer.
The Serpent's Kiss
Pick The Serpent's Kiss when enemies are hitting Zuri often and the fight gives sustain time to work. It is tied to Poison when auto-attacked and HP/S-style survival. The weakness is that heavy burst or boss patterns can make slow sustain less valuable.
The Bladedancer
Pick The Bladedancer when the run has Rush/Stall support and Zuri can survive long enough to exploit it. It is a hybrid self-healing route with bonus auto-attack damage and Crit/Defense scaling. If the Stall engine is missing, the route is less reliable.
Best Practical Rule
Use The Cruel for boss pressure, Serpent's Kiss for Poison sustain, and Bladedancer for Rush/Stall hybrid fights. Do not claim one is always best. The sources explicitly make Zuri context-dependent.