What is the test for a judge to determine the best interest of the child in a custody case?

Gordon v. Goertz 1996 CanLII 191 (SCC), [1996] 2 S.C.R. 27, 134 D.L.R.(4th) 321 per McLachlin J. (as she then was) at para 49: […] 5. Each case turns on its own unique circumstances. The only issue is the best interest of the child in the particular circumstances of the case. 6. The focus is on the best interests of the child, not the interests and rights of the parents. 7. More particularly the judge should consider, inter alia: (a) the existing custody arrangement and relationship between the child and the custodial parent; (b) the existing access arrangement and the relationship between the child and the access parent; (c) the desirability of maximizing contact between the child and both parents; (d) the views of the child;

McLachlin J. (as she then was) articulated the best interests test in Young v. Young 1993 CanLII 34 (SCC), [1993] 4 S.C.R. 3, 108 D.L.R. (4th) 193 at paragraph 206: The ultimate test in all cases is the best interests of the child. This is a positive test, encompassing a wide variety of factors. One of the factors which the judge seeking to determine what is in the best interests of the child must have regard to is the desirability of maximizing contact between the child and each parent. But in the final analysis, decisions on access must reflect what is in the best interests of the child.