Respected sir,

asically good IIT definition depends on you.

Well from my side, older IITs namely IIT Kharagpur (That's Mine :P), IIT Kanpur, IIT Bombay, IIT Delhi, IIT Roorkee, IIT Madras and IIT Guwahati are generally considered as good IITs.

For getting into any of these 7 IITS, with a decent branch such as "CS, EE, ECE(Only in IIT KGP), Mech, and others too." you need a minimum marks 200 out of 360.

This will give you a general rank under 2500 which can help you get in those 7 IITs.

Don't be so sure about the marks because last year even with 250 marks out of 504, my friends are in Mech, IIT Kharagpur.

Hello Ishan
It depends on the criteria of best engineering college in JEE Mains. Considering rank wise the top colleges in JEE Mains are to be the top NITs, PEC, IIITs. You must score about 230+ to get your prefered branches in these top colleges. (The cut-off is obviously subject to change according to the question paper which comes)
Hope this helps.
All the best ! 

Hey Ishan.

It all depends upon the question paper and the highest score. There can be no predictions about the minimum marks required to get IITs and NITs. To check out previous year's cut offs visit the link given below. You will get an idea about the cut off trends.

https://engineering.careers360.com/articles/jee-main-cutoff

Hope this serves your query. Please feel free to respond and ask further questions. 

All the best.