My verdict: the free and instant spam test.
Mailtester is perfect for a quick check before a campaign. Free, simple, efficient. The free tool to test your emails' spam score in seconds.
Mailtester analyzes your email and gives you a score out of 10 with recommendations (SPF, DKIM, content, links, etc.). It's basic but effective for occasional checks. 100% free, no account needed. Ideal for testing an email before launching a campaign. However, no continuous monitoring (for that, look at Mailreach or GlockApps).
What I like less: it's an occasional tool, no monitoring over time. Recommendations are sometimes generic. And for in-depth analyses, you need to move to paid tools.
My advice: use Mailtester systematically before launching a new campaign or when you configure a new sending domain. It's free and takes 30 seconds. For continuous monitoring of your deliverability, combine with GlockApps or Mailreach.