While there may have been some discretion over my pick of Ray Rice for most irreplaceable offensive player yesterday, I think a lot more people will agree with me in saying that Haloti Ngata is our most irreplaceable defensive player. In fact, I'm going to go out on a limb here and say he is arguably the best defensive player in the NFL.
The reason why Ngata is the choice here, is because almost all defensive schemes come down to getting penetration by the defensive-line and controlling the gaps. Ngata does such an excellent job at both things, and his contributions make the entire defense's jobs easier.
An example of this can be found in your most basic coverage scheme; the Cover 1. The general goal of the Cover 1 is to collapse the pocket quickly giving the QB little time to make a decision, while the LBs and DBs engage in man-to-man coverage.
When you have a player like Ngata who can just blow up the line on every single play, you make it so that DBs and LBs don't have to cover as long because the QB is forced to cycle through his options faster and release the ball quickly (or be sacked). This helps reduce how far a play can develop, and is especially helpful when you don't have the fastest LBs. Also, when you are getting good penetration by the defensive line, you can be more flexible with the assignment of the safety who isn't dropped back into deep coverage.
Another way Haloti Ngata benefits the entire defense is the way he draws blockers off of the LB corps because he demands respect. When you have a NT who requires 2 (and sometimes 3) blockers, you make it so the LBs have more time to attack the gaps and stuff the run. This type of contribution has been crucial to Ray Lewis late in his career.
In addition to helping the defense as a whole, the intensity Ngata plays with is just amazing. He plays like a Jonathan Ogden on the defensive side of the ball. He'll just demoralize the offense completely, and just walk back to the huddle to get ready to embarrass them some more! He never lets up, and never takes a play off; he plays like a Raven. In addition to everything stated above, Ngata is just entering his prime! So we get to enjoy this baby-eating monster for many more years to come!