There are lots of good spots. I've encountered dozens of TF's in my SH3 careers. I've sunk over 20 battleships, but have lost count. Only twice have I encountered a fleet carrier at sea, sinking none (aside from those found in port). Hunter killer groups are plentiful. I play at 88%, with only MCU enabled in case that matters. I've never opened up the campaign files to see where they are, I'd rather discover them on my own.
Among the best grids I've found for task forces
BE38, AF77, CJ75, CG43, CG95, CN22, AM42, AM33. Of course the TF's are tavelling a route, so any grid along the route is as good as any other, but I've sunk multiple capital ships in each of those listed. CJ75 is my best grid with many BBs and a few cruisers.
For HK groups, BE66/65 is the best spot, with more I've encountered west of Gibraltar, around CG87/88.
Here's a thread that lists many of these contacts (date, grid, result)
http://forums.frugalsworld.com/vbb/s...ad.php?t=86892
For the hell of it, I decided some time ago to do a career where I only attacked warships. I did reports for each patrol, which includes all the info on each encounter and the attack (if any) This can be read here
http://forums.frugalsworld.com/vbb/showthread.php?t=99279 Beware, many images.
At least 5 BBs were sunk in that career.
All of these contacts were in a stock SH3 campaign, no campaign mods aside from SH3 Commander.