First - how did Dumbledore even make the list? He was not very powerful and in the end was dead.
I would put the others on par with each other. Both are magically mighty individuals in service of a greater power set to maintain an order and prevent disaster. Now, the magic we see them use has certainly been much more developed in ELs case, and FR in general seems to be more magic laden - but the Istari are uniquely powerful in their world while El is just one among many.
Choosing not to extend ones life indefinitely does not determine the level of power an individual holds. Dumbledore appeared to me to be a very wise and learned individual with an insatiable curiosity for all things magical (including history and creatures of which was different from his colleagues). Whether this translated into raw magical power is unknown because the books never detailed any of his magic without him having the elder wand which admittedly he won from grindelwald but their relationship may have predetermined the outcome.
Power seems an odd thing to measure a wizard against when gandalf appears to have been a middling wizard but achieved great world changing things by virtue of his knowledge and skill reading/predicting people and their reactions often many years into the future (perhaps aided by some kind of foresight ability, perhaps not).
Dumbledore was of the same type as gandalf. He predicted and manipulated the actions of everyone he came into contact with (not necessarily with evil intent, sometimes people were manipulated merely into achieving their potential). He also made sure to know all he could about magic, both good and bad, which helped him greatly in fighting voldemort.
Elminster I don't know about because I've never read the novels. He seems both less and more wise (acting rashly and yet able to outwit all his foes).