This is very difficult question. There are different features to compare. Possibly as average the heaviest and most powerful eagle is the Harpy Eagle. The record for this species is a captive female, "Jezebel", weighed 12.3 kg (26 lb), but the average weight for the females is about 7.5 kg (16.5 lb).
On other hand as heaviest eagle often is cited (Wikipedia) to be the Steller's Sea Eagle. There was reported (unverified) wild female Steller's Sea Eagle with weight 12.7 kg (28 lb), but on average, females weight from 6.8 to 9 kg (15 to 20 lb).
The Philippine Eagle has the largest wingspan, average weighs about 7 kg (15.5 lb), that may mean that is a little bit lighter than the species above.
The Crowned Eagle also often is regarded as most powerful eagle. Is smaller than all above but he can lift in the air up to four times his weight.