Best platform for mock interviews?

Interviewing.io is currently the best in place to go if you just want 1 to 3 or so mock interviews and nothing else. If you plan on paying for 5 or more, I would consider an interview prep program like Formation.dev (disclosure co-founder), Interview Kickstart or Pathrise (I've removed Outco because they haven't accepted applications in a month. Exponent bought PRAAMP (which is free peer to peer mocks) and they offer paid mocks in a number of areas as an upsell but they don't have as many or as a strong mock interviewers as Interviewing.io or the interview prep programs. If you can get a good deal on them then doing a few there is probably fine. A number of people might recommend various free options, Codesmith also offers alumni free mock interviews for life, but not all mock interviews are the same. If you aren't doing a mock with a person who has either done hundreds of interviews themselves or at least been interview trained at one or more top companies, then you are losing out of the wisdom you could get. The most valuable aspect of peer mocks is to see what it feels like to be on the interviewer seat a little bit in conducting them yourself.