Both were interesting characters, and despite having more or less the same role within the show - the telepath - were distinctly different in character. I agree with Colin that the relationship between Talia and Ivanova was a good storyline, evolving from the played-for-laughs scenes of Ivanova avoiding Talia, to the revelations of Ivanova's mother and her telepathic secret. But any storyline involving Ivanova was usually a strong one - she was my fave character, with Garibaldi coming in second.
Lyta was also an interesting character, with her Vorlon links and the fact that she was possibly the strongest human telepath ever, her battles of wits with Bester and her discovery of herself with Byron's help.
I'd say Lyta edges it for me..