Mine is my home country, and not far from where I lived when I started geocaching (although I've since moved hundreds of miles away).
It was a 4.5D/5.0T cache way out in the woods where we had to hike for a while and even cross some water by walking across a narrow fallen tree. It took a lot of effort to get to the cache, and then we were rewarded with a huge container that had lots of "swag" inside. It was only the 9th cache I'd ever found too, found on my 4th ever day of caching (according to this site), so that was probably what got me "hooked" on geocaching.
However, my second favorite cache was abroad. In Brussels, Belgium there is a cache in the middle of the city, in a busy alley-way with lots of shops and restaurants nearby - it is a black box screwed onto the side of a building, right out in the open. You have to put in a combination of numbers in order to unlock it. If you didn't know any better, you'd probably just assume it was either someone's mail box or a utility box, and never pay any attention to it. In fact, thousands of people probably walk right by it every day without ever muggling it. I like a lot of aspects of that cache - the camouflage, the unlocking mechanism, the size of the cache (especially considering the location)... a great cache!