Whether one wants to speak of "best", I'm not sure. I think "toughest" or "deepest" might be appropriate.
I think the wins over tough teams in preseason by the conference pretty much set the scale for how the conference's intra-league battles help/hurt the teams' RPI. The RPI, in the end, will have some influence on rankings and on seeding in the NCAA tourney, and that makes a difference.
Right now, the Dec 19th
NCAA RPI has
PAC-12 teams (95-24) 4(#7 Ore 9-1), 5 (#8 Sta 8-1), 6 (#14 Cal 9-0), 24(#17 ASU 8-2), 29(UCLA 6-5), 32(Col 9-1), 56 (#RV USC 9-1), 62 (Wa 7-4), 91(#11 OrSt 8-2), 94(#RV Utah 9-0), 104(Az 9-1), 167 (WaSt 4-6)
ACC (127-36) 1(#2 ND 9-1), 3 (#10 NC St 11-0), 11 (#3 Louisville 11-0), 13 (#15 Syracuse 9-2), 18 (#RV Fl St 10-1), 25(#24 Miami 9-2), 31 (#RV Va Tech 10-0), 33 (NC 8-4), 41 (Duke 6-3), 66 (Wake Forest 8-4), 72 (GaTech 8-2), 100 (Clemsen 8-4), 115 (Pitt 7-5), 117 (Bos Col 9-2), 161 (Va 4-6),
Big 12 at 7, 12, 60, 61, 63, 87, 110, 166, 189, 229
Big Ten at 8, 15, 26, 36, 44, 53, 67, 77, 81, 101, 112, 124, 127, 145
SEC at 16, 17, 19, 34, 38, 49, 55, 78, 85, 106, 141, 170, 185, 240
While the PAC-12 has a slightly better Win-Loss record (79.8%) than the ACC (77.9%), clearly the RPI rankings favor the ACC.
(Aligning the ACC vs PAC-12 RPI rankings: 1v4, 3v5, 11v6, 13v24, 18v29, 25v32, 31v56, ...)
Frustratingly for me, the ACC has 6 of 15 teams with more than 2 losses. The PAC-12 has 2 of 12. So despite having fewer teams, the PAC-12 has more teams (10 vs 9) that have no worse record than 80% wins. There will be more tough games in the PAC-12 than the ACC.
Note the terrible #91 RPI ranking for #11 Oregon State who lost to ND and Texas A&M, though they beat So Carolina and thumped St. Mary's. Then, after these particular RPIs were out, they beat Duke (at RealTimeRPI, that made them rise 36 places).
But even if Oregon State were to be where it should be, it looks like ACC is still leading in RPI.
So, the more the ACC plays among themselves, their RPI will rise more than the PAC-12. But, likewise, the PAC-12 will benefit more than the remaining conferences.
Gads, I might cheer for Cal over UConn. UConn has looked beatable. But Cal had to go to overtime to beat St. Mary's. I don't know....
But first, I want Stanford to do well against Buffalo. Go Card!