Sikkim.. Wow what a place. I visited there in 2001 and was deeply exalted with the landscape there. I found it even better than what I saw in Kashmir when I went there in 2000. But since I went to both place in different seasons, a comparison would be unfair and unnecessary.
But what made me feel Sikkim better than Kashmir ? Probably one of the reason is that since I went to Kashmir just after the Kargil war, I was frightened to my guts. And in that insecurity complex, I did not interacted much with locals there and preferred to stay always in eyesight of BSF or army's patrol. Also as out tour guide told me that as an environmental effect of Kargil war, the snowfall was less and at places it even had a black snowfall.
Anyway.. Coming to Sikkim, wonderful people, and since the place is not very commercially (wrt tourist) developed, the locals treated us more like guests rather than commercial opportunity. Also the most beautiful sight was the houses of locals. They were build on hilly terrain and ever house had a cute window with curtains. Probably that's their way to keep their house warm but that was awesome to see.
The ride from gangtok to Tsangu lake was a true delight. Since it was also the way to Nathula mountain Pass which is our border with China, there were many military camps scattered on the way in between the green clad mountains. Also there was fresh snowfall the night before, which augmented the beauty. And was the Tsangu lake. What treat was it, the last surrounded by two hills, one fully snow clad and other fully green. The sun was the architect of this colorful landscape.

Other than it there were lot of Buddhist monasteries adding spirituality in the mountains. It provided a inner clam when in one of the monastery with lots of flags written with some religious script.
Sikkim is my favorite place on earth among all I have seen. Let me check out the Lord Of the Rings mountains of New Zealand and the Swiss Alps and I will update my list.