BigISP<-->SmallISP peerings

all the Real Big ISPs use a technique of routing called 'closest exit'.

They do, and the effect is that they then wash their hands of the
congestion problems that occur. Furthest exit would give a provider
the most control over quality of service but would be expensive*.
Best exit (perhaps closest unless there is congestion, then furthest)
doesn't seem possible with current routing technology.

* - but many might be willing to pay for it