Neutral:
* Rambler: Someone who goes off on tangents and talks in a rambling, incoherent way.
* Chatterbox: Someone who talks excessively, often without saying anything meaningful.
* Blabbermouth: Someone who talks too much and reveals secrets or gossip.
* Windbag: Someone who talks at length about things that are unimportant or uninteresting.
* Motormouth: Someone who talks very quickly and nonstop.
Negative:
* Babbler: Someone who talks in a foolish or senseless way.
* Prattle: Someone who talks childishly or nonsensically.
* Nutter: Someone who talks or behaves in a crazy or irrational way.
* Loony: Similar to "nutter," but often used in a humorous way.
* Crackpot: Someone who holds strange or eccentric beliefs.
* Lunatic: Someone who is mentally deranged or insane. (This term can be offensive and is not always accurate.)
Other:
* Philosopher: This can be a playful way to describe someone who talks a lot but doesn't make much sense.
* Dreamer: Someone who is detached from reality and talks about unrealistic or fantastical things.
The best word to use will depend on the specific situation and your relationship with the person.