Inkscape (exported as pdf) . In the poster you can zoom in as far as you want into this graphic and will never see a pixel (this is not the case in the above shown jpeg, though).
The problem with LaTeX is that some things tend to be really difficult even when they are really easy in direct editors. For example, to get the header in front of the logo, I had to play around with TikZ. And it took me quite some time to figure out how to have text and figure next to each other in one of those boxes (e.g. in the Introduction). The code I finally used looks like this
Analysis of excited states in large systems.
\item Reconstruct an effective ...