These are all wildly different services, but somewhat diff in their best use.
Here's a fun thing I learned from one of Marketo's deliverability experts who used to work at Return Path. Return Path is very consumer email centric. We were checking our Sender Score with them to monitor the health of our dedicated IP, but our list is heavily B2B with lots of Outlook & Proofpoint users, so the deliverability consultant said the Return Path Sender Score doesn't give us an accurate picture. We will likely continue to use it as one component of our deliverability monitoring, but will be placing less stock in it going forward.
Litmus has an amazing community and we have been using their testing tools for years and have been really happy with them.