Honestly, if you're just getting into Mishima, the temptation is to go straight for 'The Sea of Fertility' because it's his big, famous final work, but that's a marathon. I'd say the real gateway is 'The Sound of Waves'. It's short, incredibly beautiful, and feels almost like a myth. It gives you his lush prose and his obsession with purity and beauty without the heavier philosophical or political weight.
Then, if that clicks, jump to 'The Temple of the Golden Pavilion'. That's where you get the full, intense, conflicted Mishima—the beauty and destruction, the psychological torment. It's based on a real event, and you can feel him working through his own ideas about aesthetics and violence. After those two, you're ready to tackle the 'Sea of Fertility' tetralogy, starting with 'Spring Snow'. Starting with the epic right away can feel overwhelming, like trying to run before you can walk.