Floral gins always come off strange in Negronis for me, and the cucumber note in Hendrick's really clashes. The first Negroni I ever had was with Hendrick's and I didn't want to finish it. A lot of the modern style gins suffer from this, they're extremely hit or miss in different cocktails.
I like juniper forward traditional gins for a Negroni. Plymouth works nicely. I like Broker's a lot, especially at the price point. I like Punt e Mes a lot, really layers on the bitterness. I've also been using Dolin recently.
I also don't like Carpano Antica for Negroni. The sweetness and vanilla gets the whole drink out of wack,. even with a strong gin.
Anyone try Alessio vermouth? Saw it on the shelf the other day. Supposed to be another amaro-like vermouth, has wormwood in it. Might give it a try.