Which are the best Marvel DC crossover comics?

If you want to read Marvel/DC crossover comics in chronological order then I'd suggest that you read them all. You haven't mentioned that if you are looking for just crossover comics or crossover events. Here goes an official link (many good crossovers are missing though): DC Marvel Crossovers. I would personally suggest you Spawn/Batman, Superman/Tarzan - Sons of the Jungle and JLA/Avengers.

Why is NASA going to the sun?

Because there is so much about the Sun that we don't understand, and we can only acquire that understanding by getting up close, about 4 million miles close.Why is this important? Well, because solar storms can knock out power grids. In 1989, for example, 6 million people in Quebec were left without power after a solar storm.

Who is the biggest fan following in Bollywood?

According to me, the bollywood actor who has the largest fanbase not only in India but also on an international platform is undoubtedly Shahrukh Khan.Herein follows a mountain of evidence that, taken together, clearly demonstrates that India's SRK continues to reign supreme as a megastar among megastars.SRK's Twitter following is undeniably kingly in size.

Why did they introduce Captain Marvel if Thor is already powerful enough to beat Thanos?

Dirk Hooper gave a fantastic answer.I would supplement his response with some further context.Back in the 90s, as Marvel was still an independent studio trying to launch itself off the ground, and distance itself from low-budget attempts that went nowhere: it