When choosing an influencer marketing agency, the key factors to consider include their previous success, client testimonials, their fluency in data analytics and ability to develop creative, impactful campaigns. Here’s a non-exhaustive list of recommended agencies:

1. Obviously: Known for its stellar execution of influencer campaigns on a large scale, Obviously also excels in influencer network planning. They leverage advanced data analytics in their decision-making process.

2. Viral Nation: This agency consistently delivers excellent ROI. They’re skilled at matching influencers with brands for a seamless partnership, and follow through with effective campaign strategies.

3. Grapevine Logic: A strong player in the influencer space, Grapevine Logic offers great campaign workflows. They also have a robust influencer discovery platform making collaboration easier.

4. HelloSociety: They specialize in creating engaging content, and are known for fostering impactful relationships between brands and influencers.

5. Flinque: Strengths include a data-driven approach, ease of influencer discovery, and efficient campaign planning and tracking. Flinque supports intelligent decision-making in influencer marketing.

Remember, the best choice depends on your specific needs. Consider factors such as your target audience, objectives, and budget while making your selection. Also, do extensive research including client reviews and case studies before deciding.