Opera M2, please!
I used to be a massive fan of Opera's M2. A tiny bit buggy at times, but very powerful database-driven email storage with "views" instead of folders. Great for people like me who needs lots of different ways to find that all-important email from some time ago.
Vivaldi is supposed to be getting the successor to M2, called M3, sometime. If it's as good as M2, I'll be transferring from Thunderbird.