[gnufx]

That may depend on what you mean by "best". GNU Fortran has always been portable and generally decently competitive with proprietary ones speed-wise over the years and architectures. Also often more reliable. I wish it got properly funded.

[pjmlp]

The best buck for money on HPC clusters, those supercomputers using Intel and IBM proprietary compilers, which are now based on LLVM on more recent versions, thus also need to support Fortran to finish the toolchain transition.

[gnufx]

I thought in the past was at issue. Anyway if you mean "bang for money", GCC is usually zero money, and is more reliable in my experience of research computing support. I've had long experience measuring its output. Fairly recently I ran the Polyhedron Fortran benchmarks, which seemed to be used to market proprietary compilers. In the bottom line, gfortran was essentially equal to ifort (then-current releases and roughly equivalent options on SKX). It's infinitely faster than ifort on ARM and POWER, of course. I could have improved the results for gfortran by individual attention to the codes (some of which were somewhat broken anyway). In HPC the quality of compiled code, within reason, often isn't actually the bottleneck anyway.