I guess if we're going by that classic 20% drop definition, March 2009, it is!
Since then though, I've had a few rough patches. Here's my recollection of them...
1) 2010: June/July/August: I was down at least 10% (don't have my figures with me right now, though). But then in September through the rest of the year it took off.
2) 2011: July: Lost about 15% in about a month, and for the year I was down slightly (like 0.1-0.2%). I recovered in early 2012, though.
3) 2014: Year started off great, but then I remember it seeming to fizz around July, where I hit my peak for the year. Still finished up around 5.6% for the year, though. 2015, I was up less than 2%, and I remember it bottoming out in early 2016. I don't think my NW was ever down by more than 10%, though. However, since additional investments dilute that, there might have been moments where performance-wise, I took a >10% loss. By late summer of 2016, things started to improve, and I ended up with a return of around +8.9% that year.
4) 2018: Well, January was a great month. But then Feb knocked most of that out. But then, it kept building back up, through September. At that point I was probably up around 7.5%, return-wise. But then October took away all my gains and left me about flat. Slight rebound in November, and by Christmas Eve, my return was down around 18%, from that September peak. Nice rebound by December 31 though, so I think I only finished the year down a bit under 8% (from January 1, not from September). When I ran my numbers a few days ago, I had actually "made back" almost as much for 2019's start, as I "lost" in 2018. So basically, right now, I figure this latest routing has put me back to roughly the beginning of 2018.
Of all those rough patches, I'd say the 2014-2016 period felt the worst. While it wasn't the biggest hit, financially, it just got annoying, the way it dragged out and it seemed like I wasn't getting ahead. Although, who knows...we might not be out of the woods yet, with this current one!