Ovals are harder than most people think. So is rallying.
I prefer F1 just because of personal preference. Though in recent years, it's hard to call them the "best drivers in the world" anymore because they aren't even tested on the same level.
The reason why you could say that about F1 (or used to be able to) was because of.. highest revving engines, highest G forces in a corner, huge braking stops and torquey peaky engines that'd be hard to control, especially in the wet. Some of this is still true, but some of it has gone backwards. Add to that, the tracks they race on which go more and more towards carparks (which people already discuss on here). Qualifynig is still pretty intense but in the races, I really find it hard to grade or judge talent down the field because I think that any paydriver could come in and do it, as long as they are willing to follow their pit engineers instructions and manage their race. Managing a race doesn't require the same talent as driving as fast as you can does.
Ovals aren't necessarily easier, just a different discipline. I don't enjoy them as much so for me personally, they'd be "harder". A lot of road racing specialists struggle with them because it's a different set of skills and a different feel. I love tracks like Monaco, Imola, Hungary, Spa and Suzuka so while challenging, it's easier to get passionate about them. If someone was passionate about ovals, maybe they'd find them easier and find road racing harder. It doesn't mean they ARE harder though.
Just different.
I've tried to avoid mentioning the obvious cultural/nationality gap in terms of which side of the world thinks one is better and which one is worse. Since I am from neither sides of those worlds, my opinion is more balanced. 
I guess it's most about what you prefer and then your opinion will be shaped from there. It's hard to find a driver who has excelled at ovals, rallying AND road racing. If there was, maybe he'd be the one to ask. Rallying might be the hardest in terms of talent but I love that in road racing you keep doing the same lap over and over and get into a rhythm getting closer and closer to the edge. Getting "into the groove" over and over again.