This question has infitite answers! (But I'm a little biased towards neuroscience since that's what my PhD was in 😀 )
Anyway, I agree with the above posters who said that for an undergrad, finding a mentor is the most important thing. But I guess you need to narrow things down to start looking for a mentor... Personally, I think all neurosciecne topics are interesting. Besides the topic, something you might want to consider is the type of work you'd be doing. Neuroscience is very multidisciplinary. Some labs do mostly molecular work or cell culture, others do electrophysiology, immunocytochemistry, behavioral experiments, bioinformatics, neuroanatomy, etc. I mean, two different labs can be studying the same thing, but employ entirely different approaches. A student in an electrophysiology lab is going to have a much different experience than a student in a molecular lab. But, as was said above, take what you can get, and if you have a good mentor, you'll have a good experience and learn a lot no matter what the topic is.
Good luck!