OK here goes
The GC had a PowerPC based CPU with a faster clock and faster system memory than the PS2. The XBOX had the fastest clock but was based of the Intel pentium 3 line.
Gamecube
486 MHz IBM PowerPC 750CXe based core
XBOX
733 MHz Intel Custom Pentium III
PS2
297.4 MHz MIPS based processor
The speeds do not tell the whole story as a 450 MHz G4 could do a lot things faster than a 1 GHz Pentium 3 but the Pentium 3 could handle stuff like Compression Decompression better than the G4 because of raw speed. than there si whole RISC versus CISC stuff
Arguably the PS2 had the weakest CPU but it also the hardest to comment on since it is a custom(as the are the other consoles) CPU but it is a RISC based processor like the GCs but it is probably closest to the R12000 and because of teh reduced pipeline restriction is capable of higher output than pentiums at twice there speed.
Going by MHz is really old stuff the architectures matter morewhen they are vastly different.