The architect that most accurately captures the spirit of the Art Nouveau (or Jugendstil) is, in my opinion, Victor Horta. Horta was born in 1861 in Ghent (Belgium) and started his carreer as a interior designer in Montmartre, Paris. After his father’s death he moved to Brussels, where he graduated at the Academy of Arts and received his first gold medal for his art.
A year after graduating he started his own bureau, entered many contests and networked a lot, which paid off because Horta became a very popular artist. His design for the Hotel Tassel in 1893 is generally seen as the start of the Art Nouveau. He introduced many new concepts in architecture, which are still used today, for example the bel étage and the soutterain.
Due to copyright issues, not a lot of Horta architecture pictures are online (and you can’t take pictures in the museum!) So if you’re ever visiting Brussels, checking out some Horta buildings will be definately worth the effort!
The famous staircase in Hotel Tassel
The entrance of the Hotel Tassel
Rue Americaine
The website of the Horta museum, with lots of info.