I never actively seek to destroy taskforces, they really are not worth the risk, especially in a DiD campaign. The only time I will attack one is if I chance into the path of one and the hard work of positioning is already done, I'm in a sub that hasn't already had two shades of s**t knocked out of it, and I've got at least 100 meters below me. This situation is rare but I've taken out a few Bogue Carriers and a couple of Southampton classes due to chance encounters, one in fog, ship spotted at 800 meters...
I also took down the Nelson, but that was when I first started playing, at less than 60% difficulty, so I don't count that one. My favorite TF that I WILL always go for though is the one I always seem to run into early war, north of Scapa is the 1 destroyer, 2 auxiliary cruisers set up. Normally cruising along at 12 knots it's relatively easy to get ahead of and you only have to evade 1 destroyer. I only throw ordnance at destroyers in self defense if evading techniques fail me, or if there's only one guarding a convoy. I use GWX and Community Units so a lot of merchies, even early war are armed so ploughing into the middle of them deckgun blazing is normally suicidal.