What is the best Digital marketing tool for Small Businesses ? What criteria are important to select digital marketing tools?

Networking is key. It's really your number one tool in the kit. It's all about who you know and the connections you can make to take you and your business places. Being yourself and letting your personality shine through is also very important to a business. People like connecting with people not businesses or companies. Social media is a great outreach tool to use to make connections with influencers and others. Helping to answer their questions or solving their problems will yield great results. Here are a few lists of tools: 

• 20 Must Have Tools For Small Businesses - Forbes
• 10 Essential Web-Based Tools For Small Businesses - Forbes
• 24 Must-Have Social Media Marketing Tools  | Social Media Examiner
• Best online small business tools : Under30CEO
• Best Social Media Management Tools for Small Business Owners

Some are repeats, but I hope these help.

Several choices to name a few Hootsuite,Marketo,rival iq Mailchimp,surveymonkey and ofcourse your friend in online world:- range of products from google(planning tools like adwords helps to provide insights on competition ad spends and market buzz words to accomplish your goals) are some,which can be used to your advantage.