The depth of the conference should give the SEC the #1 nod. The Pac10 has the top few teams annually in Stanford, USC, UCLA, however the overall quality is equivalent to a football comparison for both the PAC10 and SEC.
At the turn of the century the SEC was the only conference in history in any sport to have all 12 members reach the NCAA tournament. This happened several times. Also, when looking at the top 10 annually the SEC dominates and generally has 30-50% of the quarter-finalists in the team competition.
If you are looking for the best college tennis programs then your analysis is spot on, however best conference should be the SEC in my opinion.