The "best" route? So you want the quickest, easiest and cheapest? Hmmm...
We don't know the time of travel, but I would say bus for the easiest affordable option (£1.40 on Oyster).
We don't know how big the bags and rucksacks are or what the limitations are with them: are you saying you find it hard to walk with them? or are you saying you find it hard to negotiate escalators or stairs? or both?
If you want to avoid escalators, but don't mind a walk, walk to Euston Square and take the Met. That's probably quickest.
If you want to minimise walking, and are happy to take escalators, but don't want the quickest journey time, then take Northern to King's Cross, and use the lifts there to get to the Met.
The walk from Euston to Euston Square isn't far, neither is the walk from Moorgate to Liverpool St. You don't have to do either walk but one of those walks is required if you don't want to change trains.
You'll need to make a decision yourself as to which is, as you say, "best" and that will have to be a compromise. Without knowing your precise requirements I couldn't say which one is "best" and my opinion may vary from yours.