Honestly, Discus are no where near being a beginner's fish. They are way too sensitive, way too expensive, and require way to acidic water conditions. The majority of fishkeepers would agree that they are more for the advanced hobbyist.
If anything I'd say goldfish are the ultimate beginner's fish. If you are talking about cichlids, I'd say convicts or firemouths or even a species tank full of angels. But since you are asking about a 125, you have alot of options. Just about every CA you can think of besides Umbies, Dovis, and Bay Snooks.
Personally, I think a 125 is a lil bit too impractical for just one fish, so see if you can atleast put a nice pleco and maybe a few cons in there with whatever 12+ inch brute you choose. Good luck.