I found the most firmware options with the Netgear R7800 (Nighthawk X4S R7800-100NAS): Voxel, Kong, Hnyman and DD-WRT. It's running on Qualcomm Atheros hardware (unlike the X6 R8000 disaster that is Broadcom, which davidc502 mentioned). If you can find one used for 100 USD or less, go for it. For the record I have WRT3200ACM and R7800.