I guess what I'm getting at is that in order for it to actually mean anything, one would have to model and *entire planet's* eco-system, geology, geography and climatology. The closest any of those came to that is Dune, be even Herbert wasn't *that* hung up on the details.
Still the major factor here isn't really heat, it's humidity. With dry air a human can survive temperatures up to about 70°C over the long term (though they'd still need regular access cooler environs and of course, water.) A more tropical level of humidity and that temperature of survivability drops by about 20 degrees. At high saturation levels, 35 °C is about the limit of liveability and 48 °C or above for any considerable length of time is death. To put that difference in perspective; with dry air a person (speaking in wide generalities of course) wouldn't necessarily die until it got up to something close to 120°C! (Twenty degrees above the boiling point of water, for the metrically disinclined.) So (somewhat counterintuitively) the dryer the air, the more survivable the heat, right?
Now Vulcan, Tatooine, and Arakkis are all depicted as very hot environments, but how arid are they?
Well Vulcan has clouds, open bodies of water, jungle biomes, and it seems a fair portion of the deserts aren't natural but the result of millennia of warfare, nuclear or otherwise (plus whatever industrial period preceded it.) So it may be the cooler of the three, it may be the more hostile to human life, which seems to track with human attitudes towards it and a number of physiological adaptations of the native Vulcans.
Tatooine has no open bodies of water of any kind, and the prevalence of moisture farming would indicate that the majority of the ecosphere's water is atmospheric (there may be subterranean aquifers but they would either be insufficient for the needs of even the very sparse populous, or else drills would dot the landscape instead of vapourators.) On the other hand, the are *some* visible clouds, so there's at least some degree of humidity, which means some kind of water cycle from the surface. Indeed, flash floods may be a (very dangerous) thing when one of those clouds actually bursts.
Now Arrakis is as dry as them come. No rain, no clouds that aren't made of dust that'll cut flesh to the bone. Almost all of the surface water has been systematically removed and supressed deep beneath the bedrock over millions of years by the larval sandtrout, because that shit is both toxic chemically corrosive to it's adult form (they literally melt and vaporise into blue smoke.) The native Fremen have been so affected by the paucity of moisture that not only does it permeate their culture, it's altered their stock human biology. Fremen *cannot* shed tears. The phenomenon is so rare it's looked at as almost supernatural and perceived with a kind of religious awe. I'd say it'd be up there with walking on the Sea of Galilee parting the Red Sea, but that would require explain to a Fremen what a sea even is, and assuming they don't just give you a look like you've spent too much time staring at the sun, they'd be more impressed by THAT than anything done to it by some beardy bloke in sandals. The other major adaption the Fremen possess is that their blood is borderline hypercoagulative; when wounded it clots almost instantly, loosing as little moisture as possible.
So what does all this mean? That Arakkis may be the hottest, (probably over the 120°C limit in places) but the lack of moisture makes the higher temperature more survivable. Vulcan may support the most life but the humidity (and old radioactive fallout) means that it takes a lot less for things to get unbearable for humans. Also the wildlife will eat you. Tatooine may be the most liveable of the three, provided one stays in shelter at noon and have access to enough water...and stays away from krayt nests and sarlacc pits.
