As the VEBA Guru whose name was taken in vain a few posts ago, I know what I have done to obtain representation in cases in which I have been involved as a party. There is no "best ERISA attorney in the U.S." There are many excellent ERISA attorneys and ERISA litigators. The top law firms all have very adequate ERISA counsel and tax litigators.
I have done as David Rigby suggests, above.
1. Narrow your search to a specific area of the law. Since you indicated it involves IRS, which Section or sections of the IRC does it involve? 401(a)-qualified plans? 409A-non-qualified plans? 419/419A-welfare benefit plans?
2. If it involves qualified plans, does the issue arise under 404 (deduction), 412 (actuarial), 415 (maximum benefit/contribution limitations), 411 (benefit accrual), 414 (controlled group and affiliated service group), etc.
3. Narrow your search to a specific geographic area, either the location of the client who needs to meet with the attorney or the locality where the IRS office is located.
From this point on the search becomes serious: you either need to be referred to an attorney that meets the criteria, or you can select blindly from peer ratings. Most personal referrals are to practitioners known to the referrer rather than to specialists who can best represent you. Responsible attorneys who know the local bar members will refer you on to someone better suited to the task.
For a cold beginning point, I recommend doing a search on
Martindale.com Look for attorneys in firms that have both tax controversy and ERISA attorneys. Read their websites to ascertain the focus of their practices. Select at least 3 who are AV-rated ("A" meaning-top level competence, "V" meaning ethical). Call all of them to determine whether you are comfortable with their personality, with the focus of their practice and how closely it matches your needs, and with their price structure. If possible, meet them briefly to assess your comfort level.
Then make your decision based on all input you have. Good luck.