I have two choices for you.
1. XiaoMi for their selection of phones that support custom ROM. Just make sure you get their Snapdragon version of some models since Mediatek doesn't always release their kernel sources(main component for making rom/kernel)
2. OnePlus for their wide range of custom ROM. From LineageOS into god knows what. Their devices are great with flagship like specs while cost lower than most flagship.

2 decives that I recommend from each company.
1. XiaoMi Redmi Note 4X
-A lot of ROM selection (same as custom kernel)
-Best battery life and size (1.5 day with 4000mAh battery)
2. OnePlus 3t
-Same as the Redmi Note 4
-OIS on the back camera
-16MP front camera
-Using last year flagship SoC