For me, my list of worst symptoms has been changing since I began treatment in Dec. At that time, my brain fog was my #1 complaint, both due to professional concerns and because it was so upsetting and annoying to not be able to think clearly.
Earlier, more so during my onset, paresthesias were my biggest issue. I started getting weird tingling/buzzing sensations (many would disappear and reappear in the same places, like a patch on my leg or a side of my face) long before I even sought medical attention. Eventually, the parestiesias intensified and went from curious to disruptive and even painful (burning).
Lately, at nearly 4 months into treatment, I'd say joint pain/stiffness is high on my list. It's odd to me since this was the LAST symptom to appear, I'd have thought it'd be early to disappear. Just more evidence that this disease is impossible to predict!
We had a poll about this not too long ago, you may want to check that out, too:
http://www.medhelp.org/posts/Lyme-Disease/Worst-symptom/show/757430