Smallville's worst death happened in its final episode when it killed off Tess Mercer (Cassidy Freeman). An attempt to fix it was made it in the Smallville Season 11 comic, but her return wasn't handled any better. Tess, a key character for the last three seasons of the Superman prequel series, joined the show in the beginning of season 8 after Michael Rosenbaum's exit as Lex Luthor in season 7.
Tess, who evolved from a villain into a hero, became an ally that Clark (Tom Welling) could rely on in Smallville seasons 9 and 10. Her arc in the show's tenth and final season involved a conflict with a parallel universe version of Lionel Luthor (John Glover), who crossed over to Clark's world. Lionel, who discovered that this Earth's Lex Luthor was dead, concocted a plan to revive her by putting Tess' heart in a cloned body of Lex. Instead, Lex was revived with Lionel's heart in the Smallville series finale. Following his resurrection, he confronted Tess and succeeded in killing her.
Tess' death was both an unnecessary and disappointing way to end the series. In her dying breaths, Lex told her that in killing her, he saving her from becoming like him, but Tess explained that Clark had already kept her from that fate. Since the closing minutes of the finale didn't allow time for Clark or anyone else to react to Tess' death, it had no emotional impact whatsoever on the main characters. The timing of her death prevented viewers from seeing how Clark was effected by the loss of a close friend.
Unfortunately, the only meaning that can be found in it was in Tess' last words. It was a moment that symbolized her transformation into a truly good person, and made it clear that it was Clark's influence that made this possible. The problem, though, is that by this point in the series, Tess no longer needed to prove that she had changed (because of Clark). The last two seasons had already done that. Tess and Clark were on opposing sides through much of season 8, but over time Tess began to change, and Clark realized that her desire to help him fulfill his destiny was sincere. The show recognized Tess' character development by having Clark formally induct her into Smallville's Justice League in a season 9 episode.
The Smallville Season 11 comic brought Tess back by having her consciousness uploaded into a computer. Tess was revived as a A.I. who aided the Justice League by providing input and analysis on their missions. After being given a cyborg body, Tess became Smallville's Red Tornado. Resurrecting Tess as an A.I. was certainly a weird and off-putting twist, and it came off as a bit silly to some readers. But, it was perhaps the only way the comic could reverse such a needless death. Tess developed so much in three seasons, that it was a shame that she was the only main character in season 10 to be left behind when Clark finally made the transition from the Blur to Superman.