IMO Subutai is thoroughly capable of beating Temujin, and almost anyone else in history, in a one-off battle with equal resources. Temujin was a great strategic thinker but Subutai was the best tactician the Mongols had, which is saying a lot. But the thing with Temujin is, as said above, he was maybe the greatest organizer of all time, be that in the sense of his army and administration, or in terms of getting the Mongol tribes to work together as one. Tremendous at recognizing and cultivating talent, no matter who had it, and tremendous at choosing his moments wisely. The loss of Subutai would definitely hurt the Mongol cause of expansion, but I think Temujin would beat him through superior organization, and conquer without him. Subutai was tremendous, but he was only a part of the larger Mongol machine. By the time Jelme was given to Temujin, that machine was beginning to roll, and it was probably already too late.
The best-case scenario for Subutai and the Uriankhai, then, is probably one where Temujin never appears or rises at all. I don't know if there's another person in human history who did what Temujin did as well as he did it, but I do think Subutai, if he somehow comes to lead the Uriankhai (while he was of low status so this isn't particularly likely, Temujin himself was only of relatively minor nobility, and such things did happen on the steppe), is capable of defeating any other steppe chieftain and taking the Uriankhai to at least temporary supremacy among the Mongols. From there, I'd say it's unlikely he builds the Mongols into the kind of war machine Temujin did, but then again, I'd say Temujin's machine was ASB if it didn't really exist. And I do think Subutai was an unimpeachably great tactician, easily the best of his era, and that definitely counts for something.