Crucible or Zeke's depending on what your team is needing. Crucible if your ADC is getting jumped on and you need to save them (extra heal too is nice) Zeke's is great if you are running an ADC and 1-2 AD champions in your top, jungle or mid. The 20 extra damage is going to help take anything on the map and help with team fights, and 10% life steal is huge for an aura. That bumps anyone who has build life steal to 20%-26% rather than 10%-16%.
Not warmog's like people are saying, you have enough hp built and if you go the zeke's route you will get some more hp anyway. Also Lucidity is nice, but more often than not I find Merc, or tabi the better way to go.