I'm not much for big words, but are "<font color=blue>best and easiest<font color=black>" what I've heard to be oxymoron's.
To me, it depends on how long the stuff has been under there. Also, depends on how easy the mower deck comes off - if easy, taking it off and scraping the caked-on grass out with a putty knife or wonder bar (I like) is a way. If the deck is difficult to remove (there are many brands that are), then just driving the front wheels onto a curb or ramp, and crawling under with a scraper of some kind is another way.
High pressure washers may work, as well as a regular garden hose if given enough time to 'wear' it off.
None of these are really "easy".
"best" to me is to not let it build up in the first place, before hosing it clean before it dries and cakes on.
If I mow wet grass, I clean it right away. Like I learned with a horse (when I was young), never put it back in the barn wet. I usually use the air hose after every mowing, and blow the dust and debris off (including under the deck) before putting it away. Just the rotten stink from wet, molding grass under a deck can be enough to choke one and cause one to keep the deck clean.
I got carried away here, not meaning to imply (because I don't know) that you are not taking care of your deck, but just asking an honest question.
If there are "best and easy" ways, I hope to hear about them too.