Lenses… are… expensive! Fortunately, I've done the leg work for you by testing out tons of different lenses so that you can be sure that you're spending your hard-earned money on only the best quality stuff.
I tried to create this list being as fair as possible to all brands, but unfortunately I don't know enough Sony lenses to make recommendations there. Sorry to be Nikon/Canon-centric. I believe that these are the very best lenses available for either brand.
Recommended Canon Lenses
Canon 16-35mm f/2.8L II USM Lens – This is a very well built wide-angle lens for the Canon system. Although it isn't quite as sharp as the famous Nikon 14-24mm lens, this one is still a keeper. It is sharp as a tack, fast, and has a convenient zoom range; however, I would only recommend this wide-angle lens for photographers with full frame cameras. If you own a crop sensor camera, then I’d get the 10-22mm. If you don’t have any idea what I just said, check out this previous post on the difference between wide angle and crop sensor DSLR cameras.
Canon 24-105mm f/4L IS USM AF Lens – This is the best walk-around lens that Canon offers. It has a convenient focal length for general photography purposes, and is quite sharp given the focal range. It works for landscapes when traveling, as well as outdoor portraits and much more.
Canon EF 50mm f/1.8 USM Lens – For just $100, the price is outstanding and the optical quality for that price is very good. For photographers who like to be close to the subject and don’t mind shooting primes for portraits, this lens is the obvious choice.
Canon EF 24-70mm f/2.8L USM – For professional portrait photographers, this is probably the most popular portrait lens on the planet. I personally don’t own this lens because I feel more comfortable shooting portraits with a longer focal length, but I am in the minority on this point. This list would be incomplete without this lens. Note that Canon came out with a version II of this lens, but you can save a thousand dollars by buying version one. I haven't done a side-by-side comparison of the versions.
Canon EF 85mm f/1.2 II USM Lens – This is one of the most coveted lenses for Canon portrait photographers. While I personally prefer the convenience of a zoom lens for portraits, there can be no argument that this lens is anything other than outstanding.
Canon EF 100mm f/2.8L Macro IS USM Lens – This is truly a fantastic macro lens. Perhaps its greatest feature is the silent-wave motor. The only negative to this lens is that I generally prefer to shoot at a slightly longer focal length than 100mm for macro shots, but this is perfect for any subject that won’t move away from the lens (i.e. no bugs). Canon offers a 180mm macro lens, but it is so expensive that there is nothing “outstanding” about it at the price of $1,800.
Canon EF 70-200mm f/2.8L II IS USM Lens – Many portrait photographers prefer shorter focal lengths for most portraits, but this lens is my go-to choice for portraits. I would feel quite comfortable shooting almost an entire wedding using only this lens.
Recommended Nikon Lenses
Nikon 28-300mm ED VR AF-S Lens – I know of no other lens on the market that offers such fantastic optical quality at such a low price point and with such an OUTSTANDING zoom range. Great photowalk and travel lens and I love that it is FX format. The only drawback to this lens is that I found it to be slow to focus considering the large focal range. See my full review of the Nikon 28-300mm lens here.
Nikon 14-24mm f/2.8 AF-S Lens – Incredibly wide angle, unbelievably sharp, and solidly built. It is widely (sorry for the pun) considered to be the best wide-angle landscape lens ever produced. The only drawback is that it is so wide that you can't really use filters with it. For filters, you'll have to use the Nikon 16-35mm lens.
Nikon 50mm f/1.8G AF-S Lens – This is NOT the 50mm f/1.8D that we have seen for years. This is the new, updated version of the lens that came out this year with a new silent-wave motor and improved optics that make this lens outstanding. I was thrilled when Nikon announced this lens because I never recommended entry-level Nikons since they don’t have a focus motor that was required for the old Nikon Nifty Fifty.
Nikon 24-70mm f/2.8 ED AF-S Lens – For professional portrait photographers, this is probably the most popular portrait lens on the planet. I personally don’t own this lens because I feel more comfortable shooting portraits with a longer focal length, but I am in the minority on this point. This list would be incomplete without this lens.
Nikon 70-200mm f/2.8 VR II Lens – Many portrait photographers prefer shorter focal length primes for most portraits, but this lens is my personal go-to choice for portraits. I shoot most weddings with the 70-200mm and only feel the need to switch lenses occasionally. Sharp as a tack through most of the focal range, too.
Nikon 400mm f/2.8G ED VR II AF-S Lens – I know… I know… this lens costs more than many used cars. However, it would be difficult or impossible to argue that there is a better sports lens than the Nikon 400mm f/2.8. I shot it a few months ago and was BLOWN AWAY. Can I give a more positive review of this lens?
Check the price for this lens on B&H Photo.
Best “Other” Lenses (Available in Sony mount, Nikon mount, Canon mount, etc)
Tamron AP 28-75mm f/2.8 XR ZL Di LD Aspherical Lens – The good folks at Tamron let me test out this lens a few weeks ago. This specific lens was INCREDIBLE!!! It only costs around $500 and is sharp, fast-focusing, and is a remarkably good macro lens to boot. I was focusing to within 6 inches! Seriously, this lens is a truly outstanding alternative to the Canon and Nikon 24-70mm lenses that cost FOUR TIMES more than this lens. I have no problem saying that this is the most outstanding third-party lens ever made. Here’s a link to the Nikon version of this lens. Here’s a link to the Canon version of this lens. Here’s a link to the Pentax version of this lens. Here’s a link to the Sony version of this lens.
Tokina 11-16mm f/2.8 lens – This lens makes the list for being the cheapest high-quality wide-angle lens available. Its fast aperture and advanced optics set it apart from the competition at this price point. This lens is made to fit both Canon, Nikon, and Sony DSLRs. Here’s a link to the Canon version. Or here is a link to the Nikon version. Or here is a link to the Sony version.
Sigma 50-500mm Lens – This is the only lens being included on this list that does not have drop dead amazing optics. This lens produces acceptably sharp, but not ridiculously sharp, images. It has a good autofocus and a convenient focal range, but what makes this lens outstanding is that it allows tens of thousands of hobbyist photographers to shoot wildlife and sports who otherwise would not be able to afford a true telephoto lens. Generally, wildlife/sports lenses cost well over $6,000; therefore, the availability of this lens has broken down barriers in the industry and created opportunities for photographers. Here is a link to the Canon version of this lens. Here is a link to the Nikon version of this lens. Here is a link to the Sony version of this lens.