I don't think the attitude behind enjoying drugs should be that of "I'm doing drugs, to hell with safety!" =) ...Obviously, you want to have the best time possible with feeling the least crappy after, and you don't want to mix up anything that will put you in harms ways, immediately or in the future.
I don't know the true risks of mixing MDMA and cocaine, but if you can achieve a high energy roll feeling without mixing two drugs, but just taking one- I'd go for the one chemical option. NO ONE can tell you mephedrone is safe or isn't, but that would be the thing I'd do if I was looking for a bright, clear-headed euphoric experience.