IIT

You may get enough material for JEE preparation on the various websites like topper.com, which provide enough material and you can even go for DLP courses of AKASH or BANSAL.

There is no such better college among the choices you gave. But, I would like to bring this to your knowledge that ISI is only for maths, whereas IIT is for engineering.

Absolutely, IIT is the best engineering college in the world.

It is quite tough to get IITs. Please check your rank in JEE Advanced exam with the help of the link given below:
https://www.shiksha.com/b-tech/resources/jee-advanced-rank-predictor
Then check the college closing rank by this link:
http://josaa.nic.in/Result2016/result/OpeningClosingRank.aspx

Okay Priyanshu, I can understand your situation you have two options:-
1. In my view, it is better to improve your class 12th percentage, if your results are good enough. If you are confident to improve your marks, you can drop a year.
2. Other option, is you can take proper guidance for both the exams, like studying for JEE, covers also the portion of class 12th.

Hello Ayaan,
Other than IITs, the highest salary packages were offered at BITS-Pilani. It was around 1.4 crores per annum. It was at the Goa campus of BITS-Pilani. It is a highly reputed institution in India and the best among all private universities in India. The fee is quite huge but you can have an education and placements equal to IITs.

According to the mentioned college name by you, DIT-Dehradun is a good option for engineering in terms of placement, faculty, infrastructure and laboratory, but it may depend on the branch, please confirm before admission.

Kindly use the college predictor app. Hope this will resolve your query. https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor
All the best.

Appear for GATE entrance exam. Moreover, there is JAM too. Go for both. For more details about admission process check any of the IITs & NITs website. As both has certain extra interview type things too.