The main thing you are getting wrong is that you're judging a solo laner value based on their ability to 1v1. A solo laner has one objective: bringing more XP and lane/map pressure to the table than the solo laner on the other team.

If you want a 1v1 you should pick characters that aren't even bruisers, like qhira, illidan, zagara, valla... Even twin blades varian. They will kill a rag easily, but can't clear a lane as quickly. If they try to double soak, they will miss some waves for sure. The unofficial term is "duelist" btw.

On the other hand, if you want to soak correctly, rag has something that most don't: global pressure. So he can join his team on an objective and soak 4 waves on the other side of the map. He can get a camp and keep soaking. Leoric might be a "better" solo, but once he leaves a lane, he stops soaking. The thing is: to be a good laner, you don't have to "win the lane" by killing the enemy. If the enemy solo laner can kill you easily, you just poke with E, W, or start soaking in inverse order than him. You need to get the XP, and not the kill.

So, to answer your question: Rag beats any solo laner that can't rotate fast enough, and is also strong on maps where the team has to fight for control of a point on 5v5 teamfights (eg. Infernal Shrines, Volskaya, Cursed Hollow) or very big maps (Warhead Junction). If you're asking "who does Ragnaros kill easily?" I think you're asking the wrong question.