If you were to ask 10 different people directly, you'll likely get 10 different answers.
But I'll give you something to look into. The HE400i.
I loved, loved my X2's. Thought for awhile they might be my end-game. But, I've always had the itch to hear some planars. When HiFiMAN ran the holiday sale on the HE400i, I jumped. I actually didn't like them at first. I was soo used to the X2's sound. I set them aside. A few days later, something about them called me back. Tried them again, and put them aside. This went on for a couple of weeks like that and somewhere in there I let them burn-in for 150 hrs (as per HiFIMAN's recommendation) unattended playing FM.
I started using them exclusively, and became accustomed to their accurate, tight, fast and detailed signature and I now find it hard to go back to the X2's. I still do love them though, and I'm going to keep them. There are still some types of music that they just sound right with! I'm not sure there's anything else that compares closely to them. Great bass and open soundstage. But just understand, the X2's and the HE400i's are completely different sounding headphones. We all have our tastes and like what we like. So, enjoy the hunt!