To all of those people who don't know jack about the actual story, read a damn book.
Isildur was the greatest hero of the Second Age. He saved a fruit of Nimloth the White Tree, when Sauron planned to burn it, by breaking into the King's Court while his people were being persecuted. He was badly wounded, but managed to escape, and spent the winter on the brink of death, with his wounds healing when the fruit bloomed in the spring.
He commanded three ships of the flotilla that carried the Faithful from Numenor to Middle Earth to escape the Downfall, guiding them, together with the two ships of his brother Anarion, to the Anduin River, where they founded the kingdom of Gondor. They built the city of Osgiliath on the river as their joint capital, and Isildur built Minas Ithil to guard the pass into Moria as his seat. When Sauron attacked and drove him out, he went north to his father's realm of Arnor & the Elves under Gil-Galad in Lindon to rally support, leading to the Last Alliance. He fought for ten years, having left his wife & youngest son behind in Rivendell, besieging Barad-Dur for seven years. When Sauron slew Gil-Galad and Elendil, Isildur cut the One Ring from Sauron's hand to win the war.
Yes, he failed to destroy the Ring, but the temptation and attachment of the Ring was so powerful that even the Wise feared to so much as touch it. Isildur was exposed to that danger by the very act of saving Middle Earth. He took the Ring not out of greed or lust for power, but for the memory of the father & brother he lost in the war. And despite the Ring weighing on his mind, he wrote an account and description of it that would critical to Gandalf identifying the Ring millennia later. He planted the latest iteration of the White Tree in late brother's seat that would eventually become Minas Tirith, and mentored his nephew as the new king of Gondor, before going north to see his wife and youngest son he had left behind as a baby, and to consult with Elrond about the Ring because it was starting to trouble him. Like Bilbo, he showed at least some of the fortitude to potentially reject the Ring.
When he and company, including his three eldest sons were ambushed, he sent his squire to flee with Narsil so that the sword that was broken could be preserved, but stayed and fought until his two middle sons were killed and only fled when his eldest son urged him to escape to keep the Ring from the hands of the orcs. He saw his home conquered by Sauron, had to leave his wife and baby son to fight a ten year war, lead an army on long siege in the heart of Mordor, see the greatest leaders of the free People fall, and kept fighting. He had to bury his father and brother, he had to see two of his sons die in battle beside him and leave the eldest to die in order to keep the One Ring from the Enemy.
Isildur was no more of a failure than Boromir, and fell only to the same thing that brought Boromir down.
