Cases can be made for both as Dylan's lyrical masterpiece. The opinions seem to be split down the middle. Desolation Row is longer, but It's Alright Ma is much more lyrically decipherable, and easier to understand.
Guess it just depends on which style of lyricism you prefer.

Desolation Row.

I'm going It's Alright Ma because it's much more concise, and manages to keep my attention for the whole duration.

desolation row does that for me and it could've lasted for days for all I care

^

The words are brilliant, admittedly, but I dunno, the actual MUSIC in IAM manages to keep your attention a lot more.
DR could hold my attention more if the music itself was more interesting. It's just kind of....monotonous, in a way.

Desolation Row is the better song but It's Alright Ma has the better lyrics.

Des Row is my favorite song ever, but its IAM. The themes are so much more concrete and clear, whereas the lyrics to DR are very abstract and hard to relate to, though they are brilliant. So IAM.

I haven't heard It's Alright Ma for a while, but being concrete and clear isn't a requirement for great lyrics.

It being more concrete and clear is the reason it spoke to me more and I voted for it.

Desolation Row