Frankly, they're all good, and you got all those focal lengths covered already. So go with your instincs. The 40 could be a good choice, given that it's inexpensive. I only used (and own) the 21 on that list, and can vouch for it, but the otherws are probably just as amazing.
Quote: But I now also apply a rule: I don't buy a lens without selling a lens.
I sort of agree. My rule is "don't buy a lens if you don't have a purpose for it". Macro, WR, low light, etc, are justification enough to buy a lens. But not if I got that "use" already covered, and not because I love owning lenses (which I do).
But some people like collecting stuff, be it stamps or lenses. That's reason enough for them.
Quote: Is there a possibility that you, similarly, are enjoying the debate about lenses more than the real photographic benefits they produce?
I also agree, but then again, if people enjoy it, it's as fair as enjoying a movie :P