If I was to hazard a guess a few weeks ago I'd assume one of the APB's, but there have been a few reported death's in the media recently contradictory to that (I'm not sure how many of them have been confirmed? Seemed to be flavour of the month in the UK like 'bath salts' in the US). They don't feel particularly taxing on the body and majority of people report quite benign effects.
Ethylphenidate feels really taxing on the heart, quite a lot of vasoconstriction and most batches will eat away your nose like no tomorrow. Same with methiopropamine for the vasoconstriction and cardiac strain. I took far too much of that one night and for the next 3 days every time I broke a sweat my skin felt like I was being stabbed with pins and came out in a rash where I was sweating! With both of there are people who use them in moderation and enjoy the effects YMMV.
AMT as long as you get your dosing right and treat it with proper care seems pretty safe (I wouldn't rush into this one unless you've done some reading on what to expect first). If you've not got some appropriate scales (+/- 5 or 10mg) it would be a good idea to give that one a miss (well most of them really if you've got large quantities of them). Out of all the things you've listed that one has been around the longest (more human test data). Major concerns are the same as MDMA, serotonin syndrome is a real possibility at heroic doses.
Can't speak for 5-IT or synthetic cannabinoids, never taken them.
