There are more than 30 stock agencies out there, and I will only recommend a few based on their credibility, trust-worthy, quality, and quantity of stocks available in their site.
iStockPhoto is definitely the top site to buy photos and vectors from. They are reliable and very customer-centric.The special thing about iStockPhoto is that it has many exclusive photos and vectors. Meaning, you cannot get these stocks anywhere else. Furthermore, the quality of their stocks are top notch. The only fallback is that they are more expensive compares to other agencies.
Another issue to concern about is the expiry of your credits. Your credits will expire in a year time if you do not finish using them. This has been quite a headache because these credits cannot convert back to cash.
If you are looking for the most-loved stock agency in the world, then Shutterstock is my choice. Shutterstock is loved by buyers and contributors. From my observation, there are at least 10 to 30 times more buyers at Shutterstock compares to other agencies.
They are loved by buyers because they have the biggest collection of photos and vectors. And most of their stock are NOT available at iStockPhoto because they do not meet iStockPhoto's requirement. This is not because they are low in quality, but because they are "too cheap" and contain text. A single file can contain hundreds of beautiful elements, and Shutterstock is selling them at the same price. You cannot get these files at iStockPhoto.
Apart from that, the on-demand and subscription plan are simple and easily understandable. The search is amazing as well and you can easily find the stocks you want. Whether you plan to buy one file or many files, it has the right choice for you. Shutterstock is a MUST-CONSIDER stock agency.
Why Fotolia? They are the cheapest. Yes, the pay-as-you-go and subscription plan at Fotolia are unbelievably cheap. You can also customize your credit plans and this is one of my favorite tool. Fotolia will remain as one of my top choice when considering where to buy.
The quantity of collection at Fotolia is lesser compares to Shutterstock and iStockphoto mainly because contributors contribute less here - because they sell too cheap and the royalty rates for Contributor is among the lowest too. You can get an Extended-License at Fotolia for the cheapest price in the industry because many of the contributors did not (or forget) to adjust the pricing for their Extended-License. So, many files remain as 40 credits for Extended License as oppose to 100 credits.
The only reason not to choose Fotolia is because their search engine are less accurate. If you compare search results, Shutterstock would almost beat Fotolia anytime. This is a very important criteria for designers because a good search engine saves time - a lot of time.
If you are looking for the best customer service with live support, then 123RF is the agency you are looking for. Their live support (Live Chat) is definitely quick and supportive. Whatever questions and help you need, they can answer you straight away and you don't have to wait for emails or queue.
There are two things that GraphicRiver offers to designers that you cannot get anywhere else. First, is the Photoshop file. Second, is the exclusivity of their files. Furthermore, their files are very high in quality.
GraphicRiver tags their files very cheap, and you don't have to buy credits in bulk, and so this is good for small time buyers. If you cannot find what you need elsewhere, then perhaps you should look into GraphicRiver. It may contains just what you needed.
Another great site. DepositPhotos is the rising star in microstock industry. Small time buyers love to buy here because they only have to pay very little before they can actually start buying. Their pricing is as good as Fotolia.
As a contributor myself, I can see DepositPhotos is moving into the right direction. I believe in the long run, DepositPhotos can be a top agency.
To summarize this, on whether which is the best stock agency to buy from will depend on who you are. If you are a small timer, then Fotolia, GraphicRiver, and DepositPhotos are good choices. If you are looking for high quality items, then buy from iStockPhoto. And if you are looking to buy a lot of photos/vectors, I would strongly recommend Shutterstock as it has the most choices, plus the search engine returns the most accurate result - always.