RISC workstations (SPARC, alpha, etc.) are no longer made today, now it's x86 only. What was the fastest one ever produced? Was it IBM intellistation 285? Or maybe something from SGI?
It depends on how exactly you define "workstation" and "fastest." You could modify most servers to be a workstation by putting in a graphics card and attaching a keyboard and mouse. Also, "fastest" is pretty nebulous as well, since different CPUs can be the "fastest" in different things.
But if I had to compile a list of fast RISC-based systems, look for things with these CPUs inside:
1. Fujitsu SPARC VIIIfx+. 8 cores, 2-way SMT (16 threads/chip), IMC
2. Itanium 9350. 4 cores, 2-way SMT (8 threads/chip), 1.73/1.86 GHz core, IMC, specFP2006_rate: 68/chip
3. IBM POWER7. 16 cores (2x8 cores in an MCM), 4-way SMT (64 threads/chip), 3.86 GHz, specFP2006_rate: 293/chip