Well the amount of phases doesnt really tell the whole story.
It also really depends on the particular mosfets / powerstages that are used on the boards.
And of course all the additional components in a vrm cirquit, like the voltage controller (pwm), inductors, drivers / doublers and output capacitors.
Currentlly the best AM4 X370 boards in order wenn it comes to vrm designs,
with the highest current capabillities and efficiency.
1: Asrock X370 professional Gaming. pwm: IR35201 6+2 phase mode doubled to 12+4 phases,
powered by 40A Ti nexfets.
2: Asrock X370 Taichi. Same vrm as the professional gaming.
3: Biostar X370 Racing GT7. pwm: IR35201 4+2 phase mode doubled to 8+4 phases,
powered by 60A IR powerstages.
4: Asus Crosshair 6 Hero. pwm: IR35201 in 4+2 phase mode doubled to 8+4 phases,
powered by 40A Ti-nexfets, same fets as the Asrock Taichi and professional gaming.
5: Aorus X370 Gaming K7. pwm: IR35201 in 6+2 phase mode, with only the +2 phase rails are being doubled, which results in a 6+4 phase design.
Powered by 40A IR3553 powerstages.
6: Aorus X370 Gaming 5. same vrm as the Gaming K7.
- The vrm designs on those boards i listed above, are pretty overbuild.
And shouldnt cause any issues.
If i have to recommend a board from the list, then i would say go with the Asrock X370 Taichi or Professional gaming.
Another great option is the Asus crosshair 6 Hero.
The Aorus X370 Gaming K7 would be my third recommendation.
However i think for its price point, the Asus crosshair 6 Hero or Asrock X370 Taichi are better choices.
I'm personally also not really a huge fan of Gigabyte / Aorus biosses.
The Biostar X370 Racing GT7: this particular board blew me away wenn it comes to its vrm design.
They are using really highend IR components on this particular board,
which is pretty unique for a brand like Biostar.
This is also the only AM4 board thus far with a dual bios.
But there are 2 things i dont like about it.
- The first thing is that they choose to use only 5K graded capacitors,
which is kinda an odd choice if i look at the rest of the components used on the board.
And in compairisson to the 10K and 12K graded capacitors used by the other boards on the list.
- The second thing being that it lacks on a B-clock generator.
Also not too sure about Biostar biosses.
Aorus X370 Gaming 5: same vrm as the Gaming K7 like mentioned above.
However this particular board also lacks a B-clock generator.
I hope this helps you somewhat. 