I think x64dbg (which contains a x86 and x64 interactive debugger), is the best interactive debugger available right now. Although the plugin interface isn't quite as nice as that of immunity, and because of that mona.py hasn't been ported by anyone afaik (which is a big shame imo).

But x64dbg more than makes up for it in performance (over both ollydbg and immunity) and feature completeness (multi-architecture, graph view like IDA, neat search functionality like search current region (very nice when dealing with JIT code), etc.)

If you're even a little bit into reverse engineering on Windows be sure to check x64dbg out, it blows olly and immunity out of the water imo.