When I started photography my first Pro series lens in the Canon lineup was the 24-105mm F4L which really started my Lens collection.
Next to follow in the line up was the infamous 70-200mm F2.8L and even though a series' II has superseded this one it is still an excellent lens and perfectly combined with the 24-105mm as a travel companion.
Later on after saving and skimping I bought the 16-35mm F2.8L and this is an excellent wide angle zoom for landscapes and general photography as the 35mm really exceeds especially with the aid open aperture at F2.8 and great for street photography.
Last but not least in this bad arse criminal line up is the beautiful 50mm 1.2L which is a monster when it comes to producing a fine depth of field and stunning bokeh. It is one lens I can recommend as everyone that gets into photography needs a 50mm.