I think Asim Riyaz will win the 13th season of BiggBoss.
Lets discuss why Shukla, Shehnaz, and Rashmi won't win first.
Shukla: Almost every season the BiggBoss team pumps up a certain contestant throughout the season and turns a blind eye to his/her misbehavior only to indirectly encourage more of that behavior, however the team orchestrate the downfall of that candidate in the last 4 weeks. Ultimately that almost-sure shot winner candidate becomes the 1st runner up. Last year it was Sreesanth, in season 10 it was Bani. Shukla is that contestant for this season. Though Shukla, unlike the contestants from past seasons, is very smart and it seems hard how is he going to fall in this trap and has already modified his behavior from an aggressive person to an entertainer. So there is still marginal chances of him outwitting the BB team and keeping the trophy for himself.
Shehnaz: Shehnaaz has gained the most from this show, even more than Asim has. She has already converted a big chunk of Shukla fandom to Sidnaaz fandom and will try her best to convert that chunk into Shehnaaz fandom. Still a small fraction of these Sidnaaz fans will still vote for Shukla and hence their vote banks would badly split during the finale voting. Also, her behavior with Salman Khan last weekend was unforgivable by Sallu's standards, so don't think she will win.
Rashmi: Though she is playing really well for the last 4 weeks or so, still she has a lot to wipe off her slate. Though the upcoming SIDRA angle will do more favors to her than to Shukla, she will still be seen as Asim's caretaker by the general viewing audience.
Now coming back to Asim, Asim is surely winning this season. I have always maintained that Himanshi was the main force behind Asim getting a positive makeover in front of the general viewing audience. Before Himanshi, for general BB viewers, Asim was just an aggressive chela-turned-foe of an aggressive Shukla. Asim himself had no idea about his social media popularity. Himanshi brought his softer side to focus and also gave him the right advise of focusing on his game by letting him aware that he is among the most popular contestants of this season. This changed Asim's game and slowly his social media popularity started percolating into general BB viewers too. Himanshi being discussed again and entering the house again as a guest will be the major boost for Asim. He will be more confident than ever before and is smart enough to make his opponent's weakness his strength.