I think rankings are about as useful as a compass. They can point you in the general direction but thats it. I can only speak for the hard sciences here but in my program your success is greatly affected by your research advisor. Now there is probably a good correlation of how “good” the PIs are in relation to the school’s ranking but Im pretty sure that if you were at a high ranked school but your PI was sub par for one reason or another you wouldnt gain all the benefits you may have if you went a slightly lower ranked school with a better PI. I have no first hand experience but I would also think the general environment of the school may be different at different ranking tiers. I am thinking that the higher ranked schools are more likely to have a competitive environment but that is something that a student would have to learn from visits or talking to grad students who are currently there. In short look for research that interests you with a PI that you feel suits your needs in a department that has enough resources. Rankings will not tell you anything specific but they are based off something so dont throw them away just take them with a grain of salt.