This article is worth a read if you're not familiar with any of these techniques. And even if you are, Nielsen's suggested approach might result in changing how you spend your limited research time to maximise the usability payoff for your website or application.
In the ideal process, you'd first conduct competitive testing to get deep insights into user needs and behaviors with the class of functionality you're designing. Next, you'd proceed to parallel design to explore a wide range of solutions to this design problem. Finally, you'd go through many rounds of iterative design to polish your chosen solution to a high level of user experience quality.
Parallel & Iterative Design + Competitive Testing = High Usability - article by Jakob Nielsen