I think the phrase 'best' is a bit misleading because what works for a hardcore prepper looking for gear tips isn't the same as what a general reader wants for a gripping story. Most 'realistic survival' books I've found tend to be non-fiction, like Deep Survival by Laurence Gonzales, which dissects the psychology. For fiction, you're often trading some realism for plot.
That said, 'The Martian' by Andy Weir is technically a man-made disaster on Mars, but the problem-solving and isolation feel incredibly true-to-life. It nails the 'one person against the elements' vibe better than a lot of earthquake novels I've read. On the pandemic front, 'Station Eleven' by Emily St. John Mandel is less about the gritty survival mechanics of the flu and more about the cultural aftermath, but the early collapse scenes feel chillingly plausible.
If you want pure, brutal, 'how do we not starve' survival, 'The Road' by Cormac McCarthy is the benchmark, though the disaster is vague. The details of scavenging, finding clean water, and staying warm are rendered with such stark, unforgiving clarity that it sets a standard. It's emotionally devastating, though, so not a fun romp.
Honestly, the genre is thinner than you'd expect. I keep hoping for something with the geological accuracy of a non-fiction book wrapped in a thriller about a supervolcano, but it usually ends up as a B-movie plot. Maybe check out 'Alas, Babylon' by Pat Frank for a classic nuclear survival tale—it's dated but the community dynamics feel real.