I've only had it for a couple months (which is probably closing in on a record of longest held iem) but my favorite iem to this day is the CK10.
The comfort is absolutely amazing. There is no iem that I have tried that even comes close. Sound wise it is pretty balanced, very detailed, great separation, treble extension is amazing and pretty decent soundstage.
Strictly IEM, tough choice. I've used a lot of great products and consider many the best in different lights. To pick one is to exclude a lot of other great options. As far as a great bang for the buck, no hassle, high sound quality, excellently balanced, toss in ear and enjoy IEM, I'd lean most heavily towards the RE252. Runner ups might be the CK10 (hot top end), UM3X (cost), or IE8 (cost, dominant low end). Sometimes these choices come down to budget or personal preference in sound. Best ends up being relative. Bang for the buck value per performance level can become important. Balance and a lack of bias can become important to appease a greater range of people. The RE252 sort of comes out on top in my mind. It's a product that rivals some of the best. It's a product that requires a considerable amount more cost to beat. It's a product that's excellently balanced and broadly capable. It's a product of very high sound quality. It's a product that doesn't require outside aids like EQing or amping to sound good. Is it my absolute favorite? No, but it's a product that's much easier to step into and enjoy.
Have listened to a lot of top tier IEMs including W3, MTPG, TF10, UM3X, IE8, SE530.
I'm in a dilemma between the IE8 and the MTPG as my fav. Both of the do bass so well but I like the midbass bump and the broad soundstage of the IE8 and the overall detail of the MTPG. So far the armatures failed to impress me, the worst one is the W3 with severe sibilance issue.
Will try to get a hold of the GR8 or Ortofon to see what the new driver technology can offer.