Telstra, in the T2 float right at the ding-dong high of the dotcom bubble in 2000. Paid somewhere north of eight bucks a share, sold them two years later at around $5. Whoops.
(Even 13 years after the T2 float, it's still only trading at $4 - but at least it's throwing off a 6% div yield, which it wasn't back then.)
Edit: since we're doing "what's your longest hold?" as well, I'm still long the 120 SPY I bought in the last week of February 2009... at $72-ish. Nailed it.