MOSFET of course. But you already knew this. What you are missing is, what other constraints do you have?
600V MOSFETs that are suitable for synchronous rectification, say (i.e., Von < 1V so it's worthwhile over using a dumb diode), are very large indeed (large die, big Qg and Coss, big $). Consider SiC and GaN, if size is an issue (and cost is not, though some SiC is now undercutting Si on resistance per buck).
If cost is the issue, then you must give up something else. Thyristors are harder to use except in special cases (mains AC usually), so we can eliminate those options. That leaves IGBTs, which are indeed quite good on power per buck at modest switching frequencies. The losses are higher so you will pay some for heatsinking and hardware, and maybe other ventilation (holes, fan..), but that is still a better outcome than the huge cost and some bother of a big transistor.
Tim