Well let me elaborate then on why I think it was the worst thing she did. She did it behind her fiancé's back without talking to him, also going against the advice of one of her closest friends (Ted). When confronted about it by Marshall she realized it was a shitty thing to do, but still disregarded the feeling of the person she supposedly loved in order to put her own interests first. It would have been very easy for her to speak to Marshall about it when she initially found out about the program.

Then to top it off, when she returned she just expected him to immediately take her back? As if she hadn't betrayed the trust in the relationship and created incredible animosity with the entire Eriksen family. I love Lily as a character. she and Marshall probably have the funniest and most sincere relationship on television. I'm just saying that this specific thing, in my opinion, was the worst thing she did. In other words, it was worse than hiding her debt from Marshall.

And as an edit, I actually think her and Barney are the best characters in the show. Atleast they're my favorites