I'm a degenerate nicotine addict, so I often don't notice nicotine in pipe blends. Here are some that knock me on my ass, though:
Gawith, Hogarth & Co.: Their ropes, along with the Dark Flake/Plug/Kendal Dark/Kendal Kentucky are incredibly high octane. I really like Dark Flake Unscented a lot. Unfortunately, these are frequently out of stock online, but some B&Ms carry them as well. Available in bulk at slightly higher than normal bulk prices. Still relatively budget friendly.
C&D Old Joe Krantz: This is readily available and at good bulk prices. Sour, musty and strong as hell. When I feel that catch in my throat, I know I've just bought a one-way ticket to woozytown. Good stuff!
Any bulk Dark-Fired Kentucky blending tobacco: DFK is strong. I haven't tried it, but I see Stokkebye offers a DFK blending tobacco. It would surprise me if it didn't have a nice kick in the ass waiting for you.
As I try to ditch the cigarettes, I should be on the lookout for these as well.