Dettol.
Works on both plastic and metal equally well.
Place minis in 1:1 mix of Dettol and water for 24 hours.
Put on a pair of surgical gloves, grab toothbrush, and fill a bowl with soapy water (soap powder works better than hand soap).
Scrub. Don't worry about any slimy gunk that's loose but hard to get off, just plop the scrubbed minis in the bowl of soapy water.
Once you're done scrubbing all the minis, slosh them around in the bowl of soapy water. This will loosen up the gunk that remains. After they soak a bit, you should be able to give them another scrub and the minis will be mostly clean.
Take the soapy minis and leave them to dry. By the next day they should be fairly clean, and you'll find the really stubborn primer you couldn't remove the day before, are flaking off.
Depending on what you're dealing with, you may have to repeat the dettol soak after the first scrubbing, before going on to the soap soak.
This may not be a good solution for resin. We've tried it on some resin bases once and they came out oddly soft and Dettol-stained.
If you don't have a lot of time and cost isn't a factor, just use brake fluid. Soak in brake fluid. No scrubbing should be needed, after about two soaks you should have pretty clean minis.