Hey Jonny - I was in your shoes about six months ago and you bring up a really good question. I wish that as a community, we'd getter better at classifying things like 555 and RY4. Not only are they drastically different from vendor to vendor; but even more annoying is that some of them are EXACTLY the same. I assume its because while some make their own juices, others just source them from China.
Labeling needs to get better IMHO. I wish that manufacturers would require that Vendors specifically state a "manufactured by: " label.
Okay, now that i'm off my rant:
Backwoods Brew is currently my go to 555. They have two, a standard and an "exotic". I like em both equally and switch back and forth. Probably my fave as i type this. BWB is much more complex (as with all their juices) than other brands and its clear that Don's juices are made from scratch. He didn't just buy a "555" flavor and add it into the mix. Thus, there's much less of a fake peanutty taste than most other 555s. The standard 555 tastes kinda like tea to me on the inhale with a nice smoky tobacco going out. I can't really describe the Exotic. Definitely sweet tobacco...with hints of nuttiness. There was a time i liked 555 better than the exotic, than i switched. Now i'm back again. (Housebrew).
Smokeless Image and V4L(wowboy juice) seem to be the exact same to me. I strongly suspect they're sourced from the same place. These were my go-to and what i consider 555 "should" taste like. Very nutty on the inhale; very sweet at the back of the throat. (Manufactured elsewhere).
KBV - To be fair, i got this as 50/50 and so the taste isn't quite "there." I'd need to go back and get this in a 70/30 or 80/20 mix to have more of an apples to apples comparison. I didn't love it, but there wasn't anything offensive there. (Housebrew)
ECBLends - Just awful. Either there's something wrong with me or there's something wrong with ECB. To me, all their flavors taste like it was scooped from the floor of DOW Chemicals. (Manufactured elsewhere).
MVS - I LOVE this company, just not their 555. Nothing super-offensive about their juice (be careful as their juice is sourced differently than their pre-fills), just nothing much to love about it. I didn't get the nuttiness that you get with SI & V4L. (Manufactured elsewhere).
MOV -I was actually really disappointed with this one. I really wanted to like them...super fast, great packaging, and nice quality. A lot of people like them, so it may just be me; but there is something off with their base. I just can't get that smell outta my nose. The smell is so strong to me that all of their different flavors are completely overpowered by it. It's almost medicinal. (Housebrew).
Hope this helps and i hope more people will expand on the list and or add to my opinions as i'm sure others will disagree. I also hope people will say things like....This tastes close to that, etc. so that if you have a visceral reaction to something, you won't just order more of the same.