Originally Posted by
bat39
Eobard Thawne for Barry Allen, and the Flash Family in general.
In the original timeline alone, he killed Iris, almost killed Barry's second wife Fiona, and drove Barry to become a murderer by killing him.
Then, after he was resurrected, he went back in time and killed Barry's mother, rewriting Barry's entire history and giving him a far less happier childhood. Barry's attempts to undo Thawne's changes also led to Flashpoint which, while not the only culprit for the rewriting of DC history (we have Doc Manhattan to thank for that!) was certainly a contributing factor.
This rewrite to DC history erased Barry's relationship and marriage with Iris (and their kids and grandkids from the future), sent Wally West into limbo for a while, and erased Wally's relationship and marriage with Linda, as well as erasing their kids from existence!
And I'm definitely missing out on some of the more recent damage he's done...