Well, first of all, SRK wasn't a top star from the 1990s until 2007. Unlike today, stars in the 90s were more or less on equal footing. In the early 90s, you had Sanjay Dutt, Jackie Shroff, Anil Kapoor, Sunny Deol, Govinda - who were more or less dominating the scene. Then, came Aamir, Salman, and SRK - the new comers. By mid-1990s, Ajay, Sunil, and Akshay had joined the list, but Jackie and Sanju faded away quickly, and Anil wasn't as big of a draw anymore.
It was now down to Sunny, Govinda, the Khans, Ajay, Akshay, and Sunil (for a brief period). But then from 1995 onward, SRK and Salman rose into prominence - challenging Sunny who was the outright number 1 star in the mid-90s. By late 90s, it was the trio of Salman, SRK, and Sunny - even though Salman had most hits out of them from 1995 until 1999, Sunny and SRK had bigger hits. So, as you can see, there wasn't a clear-cut #1 in the 1990s. Their stock fluctuated.
After 2001, Sunny Deol started fading away. Salman fell into all sorts of troubles. From 2000 onward, SRK was more consistent. With Sunny out of the running in 2002 and Salman no longer in the picture, SRK took the mantle in 2002 after Devdas. From 2002 until 2008, it was SRK clear-cut. HR arrived in 2000 and instantly became a star. If he hadn't slacked off after 2006, he'd have become #1. But that didn't happen.
In 2006, however, the multiplex boom took over. From marathons, box-office success became about sprints. The question was how much a movie could earn in 3-4 weeks now. Released in 2005, No Entry is the last big hit of the pre-multiplex era. There were multiplexes before 2006, but in 2006, the scale and dynamics changed completely.
Now, by 2006, Aamir was already making headway with Mangal Pandey with its record opening, but it was Fanaa that truly brought Aamir into the game. He was #3 by then. Akshay had a super year in 2007, so with HR slacking off, the competition was between SRK, Aamir, and Akshay - with SRK still #1. In 2008 though, things turned. Aamir delivered the first ever 100 crore movie, an ATBB at the time and followed it the next year with another ATBB in 3 Idiots. Aamir finally took over from SRK in 2009. It was Aamir, SRK, Akshay in that order.
Salman, who was not in the running for top-3, returned with Wanted - his first hit in over two years. Wanted didn't change the dynamics, but until that point, it was the biggest single screen hit since the multiplex era had begun. It wasn't all that big in the multiplexes, but what it did do is lay the foundation for Eid 2010 - a movie that would become an ATBB on the basis of sheer star-power and the charisma of the lead - Dabbang. That hadn't happened since Amitabh Bachchan's era, where the personality of the actor/star alone would make it a blockbuster.
That is essentially why, from 2010 onward, Salman became the biggest star of the country outright. A Dabbang hadn't happened since the days of Amitabh Bachchan and hasn't happened since then. Following Dabbang, we all know how the race was now between Salman and Aamir, but for the first time in his career, Salman had become the outright #1 star of India. At this moment, it's still Salman, Aamir, SRK, Akshay, and HR in that order. The distance between Salman and Aamir isn't as big as it was in 2012 after Dabbang 2, however. Another Tubelight and Aamir would comfortably cruise past Salman.
As for Prabhas, well, it's too early to say that he's the #1 star right now because Bahubali 2 is a once-in-a-generation phenomenon. If Prabhas' next movie turns to be a big hit, even a 200 crore grosser (Hindi version), I think it'd be safe to say that he'd have arrived in the Hindi film market.
0