I needed a custom flash for my K-50. Its TTL capabilities make work easier. In the photo below, I shot a candy for a friend's wedding. When it comes time to upgrade my camera body, I know this flash will be there. I've come to the conclusion that flashes and lenses should match manufacturers, it makes the integration seamless.
Nissin I40S Camera Flash Black
9 Review
YONGNUO YN660 Speedlite: Versatile Wireless Manual Flash with GN66 Power for Canon Nikon Pentax Olympus
10 Review
Godox TT685F Speedlite Flash for Fujifilm - 2.4G TTL, GN60, 1/8000S HSS, 0.1-2.s Recycle Time, 230 Full Power Flashes, 22 Power Output Steps
10 Review
Canon 600EX RT Speedlite Flash Black
10 Review