Hmmm, if I'm honest...
Graphics? Perhaps, if I'm only comparing post-Prologue Premium models and the 'Ring. PD's lighting engine is still the best on consoles, but I can't overlook the majority of the assets being so out of date.
Physics? Nope, others do it better, and the lack of any reasonable tire model is too much of a problem for me.
Car count? Looking strictly at the numbers, no, nothing has more, but I prefer quality over quantity.
Day/night cycles, weather? They do function very well, as long as I'm not running with a full field of opponents.
Photomode? Seriously... Yes. No other game's Photomode functions as well as the one PD designed. It's fantastic.
Online? Connections seem to rarely be problem-free, but the amount of options available in a room are handy. And compared to its main competitor, I certainly enjoy GT5's ability to have "open track day" rooms, where we don't have to immediately race, and that I can tune and change settings while in the room. You don't realize how handy that is until you're loading different setups before races, hoping one works for the track.
Tracks? Yes, the 'Ring has no better version on consoles, and Spa is great too. But past those, and the original GT trifecta (Deep Forest, Trial Mountain, Grand Valley), I like other track lineups better.
"The brand". GT's strongest feature may just be its reputation (deserved or otherwise). It's continued popularity means there's always a fair amount of activity online. The sheer number sold speaks to that.
So, Photomode, online options, and reputation. I'd say the first or last would be most deserving of the singular title, if I could decide.