Canon EF28-300mm f/4.5-5.6L IS USM - tele, zoom, entire zooming range
When: Always! At least almost. Its usage covers up to 80-90% of all possible situations. Outdoor and in studio. It's hard to imagine more versatile solution for beach and park. Almost macro, almost wide angle. Amount of light - modest.
Pros: Multipurpose, all-in-one. Closest focusing distance is 0.7 meters. Optical image stabilization with 2 modes: static and sport. Fast and convenient rubberized zoom control. Filter diameter is 77mm. It comes complete with a removable tripod collar which gives possibility to take camera and lens off tripod still keeping collar on tripod.
Cons: Heavy, big, and expensive.
My examples:
Canon EF100-400mm f/4.5-5.6L IS USM - tele, zoom
When: Outdoor only. For something distant. For example, it's possible to spy for neighbors. Wildlife, such as birds, animals, which will not let you come closer. With EF 1.4x extender max zoom can be extended to 560mm, but pictures on such zoom are very-very soapy. It lens is rarely used because I am not after birds or animals, and I have got EF28-300mm. But if you are headed towards national parks, this lens is just a necessity. This entire design is the same as seen on the Canon EF28-300mm described above.
Pros: Fast and convenient rubberized zoom control. Filter diameter is 77mm.
Cons: Heavy, big, and expensive. Dark. Especially on big zooms (close to 400mm) which this lens was designed and is used for (f/5.6, plus circular polarizer filter, which kills light well). AF does not work with 1.4x extender, and lens becomes darker with it.
My example:
Canon EF17-40mm f/4.0L USM - wide angle, zoom
When: Outdoor or indoor with good (or flash) light. When you need a mostly moderate wide angle. As a usage example, this lens is good in the crowd like city fairs, where distances to objects are extremely limited and it's still nice to have limited zoom. Zoom range which is 40mm here is not a zoom-zoom, i.e. not serious zoom. This lens is also ok to take panoramas by one shot only.
Pros: It's only my L lens which has black exterior. Lightweight. Filter diameter is 77mm.
Cons: (in my personal usage style context only) Limited usage because of limited zoom range.
My examples:
Canon EF50mm f/1.4 USM - no zoom
When: If there is almost no light. Indoor twilight, romantic relaxing atmosphere. When "flash photography" is not appropriate. Events where there is absolutely no any desire to bring big heavy "things". Just put ISO 800 or 400 and - ahead. It's kind of compromise between point-and-shoot camera and professional cannons. Also good for portraits.
Pros: Super-duper compact, lightweight, and bright.
Cons: (in my personal usage style context only) Because of low light conditions I usually experience lack of depth-in-view and grain (ISO 800 and 1600). But it's not a real shortcoming - other lenses just give up here.
Quote of auto engines engineer from Germany who asked to hold my Canon 10D camera with this 50mm lens: "I'm sorry, but what a crap are your cameras: It does not show picture [preview on LCD before shoot], and it even does not have a zoom!"
My examples:
Tamron SP AF 14mm F/2.8 Aspherical (IF) - ultra wide angle, no zoom
When: If there is almost no space between you and object. On very close distance. Indoor and outdoor. Interiors. Used rarely.
Pros: Much less expensive than Canon; wide angle is outstanding; trying to suppress barrel distortion; quite bright; sharp.
Cons: Still expensive, slow and loud AF, no possibility to use standard filters because of its construction (outside sphere prime glass).
My example:
Peleng 8mm F/3,8 Fisheye - 180 degrees angle
When: To amuse with amazing pictures. This is fun lens.
Pros: Inexpensive. Extremely sharp. Made in Belarus.
Cons: Completely manual! Poor packaging. Loose both caps.