Well, on a budget, the Klipsch Custom 3 is a great choice. You get balanced yet extended bass with good weight and articulation and you get good treble but slightly smoothed to hide some of the minor imperfections caused by lower bit rates. Isolation is quite good and many tips fit it including Shure Olive tips and Comply tips. The cord is a joke, but it's livable and the very light weight is actually beneficial and microphonics are very minimal.
For the $200 price point, I'm a fan of the Audio-Technica CK10 or Hi-Fi Man's RE252, and their used prices can be quite good. The RE252 is geared slightly bright so you might desire a little more bottom end presence. The CK10 is pretty neutral but with a very strong yet narrow spike around 12kHz that can make it sound pretty hot. Both are pretty tame in bass but well extended. These do represent a couple of the most musical earphones I've used, and their pricing, especially used, is pretty stellar for what they are. The downside is I expect you may still want more bass or at least something that has literally zero deficiency. The Custom 3 is a slightly better gearing for this but also a half notch down in overall SQ. I would also suggest the Monster Turbine Pro Gold as a great option, one of the flattest frequency responses I've used and well extended on both ends. Pricing may be a tad much for what they are in terms of overall sound quality, but they do very little wrong outside of being priced a little high. Bass is well controlled and well balanced. Mids are very good. Treble can sound slightly smooth which again can fit along with your concerns over low bitrate songs. Some of the other earphones, especially the RE252, is very sensitive to bitrate and overall quality and can sound downright terrible on a bad recording. The Triple.Fi 10 is an outstanding earphone and well priced and definitely a favorite of mine, but I may avoid that too since its treble end is very detailed and will not hide lower bitrate information. People also complain about fitment, but it's more so an issue of just using the right tip. A set of Comply T-500 tips certainly help. Plus they help tame the top end a little (same for the CK10).
Backing down to a couple good options:
Custom 3 - excellent price, excellent sound quality, excellent balance, light weight, doesn't need amping,
MTPG - a little pricey, excellent sound quality focusing on texture over crispness, flashy looks, good materials
I kind of go back to the Custom 3 as a great package. I'll just suggest grabbing some Shure or Comply foams for it, and they'll be super comfortable.
Isolation is relatively similar across a lot of IEMs. There are a few stellar ones and a few really terrible ones, but a lot of them are very similar. The tip choice and actually having a good seal will be the most important part. Outside of that it's just earphone construction and its ability not to transmit outside sound into the ear, and a lot of them are similar enough not to care.
