What's the ideal spacing...

... between siblings?

I know some of you wonderful ladies are expecting again (or have had another- congrats!).  If you could pick the perfect spacing between siblings, what would you select? Have them close together or space them out a few years? 

Ready for Another Baby? What's the Ideal Spacing Between Kids?- 
ttp://www.ivillage.com/navigating-age-gap-whats-ideal-spacing-between-kids/6-b-369327