Well, I suppose the "safest" (and I use that term loosely with RC opioids) is u47700, but only because the amount you need to dose is much higher than most fent analogues. u47 is mg dosing, whereas most fent are microgram.
Other than these two, there aren't any other legal (at least, gray area legal) ones out there. I'm pretty sure o-desmethyl-tramadol is already banned in most places. The only other one I can think of is mt-45 which causes peoples hair to fall out and grow back white and skin to turn scaly.
I've been trying u47 out lately, though, and I haven't seen much in it personally. It makes me feel out of it and has a tinge of dysphoria to it. There are people who say it doesn't work and those that say it works so well its instantly addictive, to the point where they consider it more addictive than heroin. Many people have compared it to hydromorphone (although, I don't see it). Several people have already OD'd, especially when combined with benzos or alcohol, despite having massive tolerances. You have to be incredibly careful with it. Do your research. Also, realize that it's never been tested in humans, only animals!
Most fent analogues are super dangerous, are essentially crack, and I hear about ODs all the time; I can't recommend them.
This is the only time I'll probably ever say you're better off with big pharma.