After Washington, of course, and speaking in terms of military figures, Nathaniel Greene immediately comes to mind as a potential "Second Least Expendable"—and that's despite his enormous blunder at Fort Washington during the New York campaign of 1776, where he convinced Washington he could hold the fort. Instead, the entire bloated garrison was surrendered, decimating the Continental army and resulting in thousands of Patriot POWs.
In "The American Revolution in the South" by Henry Lee—Robert E. Lee's father, and a superb Revolutionary officer in his own right—we see—via firsthand accounts—how Greene's masterful out-generaling of Cornwallis throughout the South was absolutely key to engineering the ultimate fate that Cornwallis met at Yorktown.
Had Greene not replaced Gates after that general's crushing defeat at Camden, the outcome in the South would likely have been equally disastrous.
Thinking in terms of the earlier part of the Revolution, I'd have to say Samuel Adams was indispensable, so he'd be another candidate for "2nd least expendable".
Nathaniel Greene was Washington’s pick to lead the southern army. The Congress, however, chose Gates due to his victory at Saratoga. After the disaster at Camden where Gates was no closer to the battlefield than thirty miles in the arrears, Congress finally listened to Washington and sent Greene south to try to scrape togethher the fragments of the southern army. Grreen has the masterful touch of being able to see each man’s greatest strengths. He chose Henry Lee, an Aristocrat from Virginia, to work with the countryman Francis Marion. It was a great match. He chose Dan Morgan to lead Cornwallis and Tarleton a merry chase while he reorganized and rebuilt the southern army. Morgan’s brilliant strategy at Cowpens decimated Tarleton’s troops and assured a much weakened Cornwallis who was finished off at Yorktown.
This is one of those impossible questions to answer, but it's good to see someone remember Samuel Adams....