Irish Flake does not strike me as all that strong; on par with Peterson's Perfect Plug (3P) which is also quite good.
For a solid medium-to-strong, the same blending house offers Irish Oak, which is a favorite.
I have smoked a fair amount of Cotton Boll Twist and it is about as strong as your average GH/SG rope. Those are pretty much the apex of nicotine overdrive, and most of them are delicious.
Royal Yacht is a shade past medium-to-strong. I smoke it mixed with Burley Flake #1, which is a wonderful high nicotine blend.
Many find heavy nicotine in Five Brothers!. Perhaps, although it seems more medium-to-strong to me. Great tobacco nonetheless.
From the SG/GH ropes, I find Brown No. 4 to be the best, truly an epic blend. I love Brown Twist Sliced/Happy Brown Bogie, but it might have too much smoke-cured leaf for the long term. Brown Irish and Black XX are also great.
Semois is another great one, but medium-to-strong, not brutally strong. Burns hot with some bite but a delicious and unique smoke. MSO489 is the expert here.
Dunhill Nightcap and its American cousin, Engine 99, both deserve some mention here for Perique-laden Englishes.
Everyone mentions Tambolaka, and it is quite good, but only as strong as a GH/SG rope, on par with Cotton Boll Twist.
Old Joe Krantz is of course the codger favorite that every OTC blend wishes it was.
1792 is great but blighted by the Lakeland Essence, try Lakeland Dark instead.
Old Dark Fired has an appeal as well, although it sort of tastes a bit over-smoked, like tourist BBQ, at times. Still I like it. A Philistine obviously.
I might speak up here for C&D Cube Cut Burley. A solid medium-to-strong, it takes on flavors very easily and is a great mixer.