Chanters were one of the classes where this used to be a big argument because theorycrafting would lead one to lean towards a playstyle choice of spec.
Abjuration is runes and some buffs. Useful, arguably life-saving (runes) but not where I specced my chanter.
Evocation is nukes. Chanter nukes are weak... waste of a spec or secondary in my view, but I definately remember people advocating it as a secondary.
Divination... does any class spec in divination? Granted they put PBAoE mezzes under it (but they were just mega nerfed), ditto for stuns of that type...
I went conjuration back in the day because I was a crowd-control chanter in a family guild needing crowd control for Ldon's and low PoP and earlier content. Mezzes and memblurs live in conjuration. If I knew I was going to get to do CC then I'd probably still go this way.
Alteration was my secondary spec (when that AA opened up) because dots and tons of buffs are there. A buff-bot chanter probably specced here. I would actually spec a chanter here today to give it a try, because I would be soloing and charms live here so I would be curious to see how it played out (I'd go conjuration for the secondary).
Note that I gave up on chanter a few years back because I found that CC was so rarely needed in the parts of the game I was playing, and CC was the main reason I had rolled the class. Hopefully some current chanter players chime in on the thread.