Basically 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.