What is the best way to retrieve data from a Sharepoint server?

I'm not really good at share point i use it for 4 month or less but as i remember, you can use Business connecitivity services check this http://technet.microsoft.com/en-us/sharepoint/ee518675.aspx hope it may help

There are following ways the SharePoint exposes the data out side: 1.
Server Side Object Model (you need to deploy a web service/WCF Service at the front end server of SharePoint Servver which uses server side object model to retrievve the list/library data from the sharepoint site and return the data in XML/your custom entities) 2.
Client Object Model This is also a good approach if your consumer is have .NET frame work 3.5 then this is the valid option 3.
use the SharePoint OOB web services (WCF services/Web Services) Advantages for #1 : cross platform, your custom security to access the service is possible Disadvantage for #2: the current logged in user should have the permission to sharepoint or you have to expose the user name and passwords Advantages for#3: Support REST Services,OOB, but disadvantages of #2 are applicable to this option.
Now, Match the requirement and take the decision.
Hope this will help..

if you are using .Net for the stand alone application you have client object model and web service for accessing data from SharePoint.
If the stand alone application is using some other language other than .Net, web service is the only approach.
BCS is used to read data from other data sources into SharePoint and not reading SharePoint data.

You Can use the BCS (Business connectivity services) in SharePoint to connect to an External data source and retrieve the information.

Retrieving data from SharePoint DB directly is not recommended, client object model is the best of what you can do

It depends upon the version of the .net platform you are developing the application on. If the platform <3.5, then you can use SharePoint dll to get the item. An added advantage here is that the dll gives you the option to convert the SharePoint lists items retrieved into a DataTable which is very useful in case of data binding eg. repeaters, gridviews

You can also use Webservice to get the data in XML format and then parse it according to your requirements.

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You can use the foll. methods depending upon your requirement:-

Server object model

Managed Client  object model

Javascript object model

REST API

PowersShell

There are different ways to get the data based on product ID:

1) CSOM/JSOM Code

2) SSOM(Server Side Object Model ): Deployment needed