So here's the deal: FrikBot X by Ryan Smith is one of the best AI opponents released for Quake. FrikBot X+ is an updated version of FrikBot X as modified by "Igor 9". The changes in FrikBot X+ were quite good, but they inadvertently removed the original FrikBot X capability of automatically cycling through multiple maps. So I added that feature back in. FrikBot X++ is just FrikBot X+ with map cycling.
Note: Using FrikBot can be more than a little confusing. If you'd like to try, you may want to have a look at the "Steam guide" that I've put together on that topic; it's kind of sprawling, but it does have a "quickstart" section and should clear up most of the common problems and questions. It also describes other bot mods that you may want to investigate, especially if you prefer QuakeWorld.