Feel free to define "best" however you wish - all I ask is you explain your choice in the comments.
I only listed books in this poll that I have access to, so there are a couple missing. I also recognize that the ranged/modern combat rules evolved through sourcebooks as well. If there's a missing book or a main book + sourcebook you feel represents the "best" to you, feel free to select "other" and explain it in the comments. Poll options are listed in the order of publication, as far as I was able to determine through the internet.
- April 1981, Mechanoids®
- September 1985, Teenage Mutant Ninja Turtles & Other Strangeness
- November 1986, Robotech® (original RPG books)
- April 1987, Heroes Unlimited™ Revised
- October 1987, Beyond the Supernatural™
- August 1990, Ninjas and Superspies™ Revised
- August 1990, Rifts® (original book)
- ?? 1993, The Compendium of Contemporary Weapons
- August 1995, Nightbane®
- March 1998, Heroes Unlimited™ Second Edition
- July 1999, Systems Failure
- December 2001, After the Bomb®
- June 2003, Chaos Earth®
- October 2004, Splicers®
- January 2005, Beyond the Supernatural™ Second Edition
- August 2005, Rifts®: Ultimate Edition
- March 2008, Robotech® Shadow Chronicles
- November 2008, Dead Reign®