When it comes to classic systems, nothing is more attractive to me than the SEGA Dreamcast for an assortment of reasons. My Dreamcast is still hooked up and used regularly but I also enjoy emulating DC as well.
According to libetro, Flycast just added Direct3D 11 renderer, which could be very useful for Microsoft users. I haven't checked it out yet, but intend to over the next few days. I'm also anxiously awaiting the arrival of my Retroid Pocket 2 Plus to see how much Dreamcast Emulation has improved from the original RP2.