Like you, I also seem to be naturally tolerant to most things. There are some things to consider when wanting strength from kava.
1) Reverse Tolerance. You may get minimal effects for a period of time and you'll only begin to feel kava at it's fullest after putting in many days or weeks of drinking first.
2) Empty Stomach. The effects come on stronger and quicker if you haven't eaten for several hours prior. A full stomach can put a major damper on the experience.
3) Strains. Different strains of kava have different dominating effects. There's also no standardization. You might buy a bag one time and feel that it was strong, then buy the same bag a year later and feel like it's weaker or different.
4) Expectations/Setting. If you go into it hoping to have a major experience, you'll probably be judging it too much to allow the good effects to pop through. If you try to drink it hard to see what it's capable of, you might end up sick before ever enjoying it. If you're not in the mind-state to just chill and let it be what it's gonna be, you may have a hard time truly feeling it.
Anyway, the kavas that I really enjoy the most AND I also consider strong are: Stone, GHK Mahakea, Papa Kea & Opihikao, KBR Fiji is pretty good...BKH Boroguru and Melo Melo. There are many others that I enjoy, but not so much for their strength or heaviness. A lot of people loved Solomon's but I only got mediocre (yet nice) effects from it.