No one.
I say this as someone who was never a Deepika fan, but I absolutely loved Deepika as Piku - hands down one of my favorite Bollywood movies. The thing about this film is, either you connect with it or you don't. I connected with the film (not a Bengali) and I would like to say that was all because of Deepika's portrayal of Piku. The fact that Deepika was able to subtly emote so beautifully throughout the film was everything. There are so many scenes where Deepika's eyes, or her smile or her silence, or her overall demeanor somewhere leave such a mark with you as the audience, and those scenes exactly are what made Deepika shine. Not to mention the way Deepika is able to stay true to Piku's characterization was also very commendable - for example, the way she tries keeping herself together all the time yet has moments of obnoxiousness, vulnerability, sadness etc all of that was so well enacted. So, no one but Deepika.
3