Why are X series eliquids the best?

After experimenting with a variety of eliquid producers and their products, I've come to the unpopular conclusion that the X series eliquids have some of the strongest flavors. I'm not sure why, but their eliquids are genuinely special and enjoyable to vape as opposed to any other big brand eliquids. I'm curious as to why this is the case. Why are X sequence eliquids so hard to come by if they're rarely available?