There are numerous past threads on this forum about this subject, but the short-skinny version of it is most are of very good quality although each has some idiosyncrasies in availability, detail, and running qualities; it is also a highly subjective evaluation.
Perennially high ranked:
Atlas: smooth drives, nice detail, not many new releases
Kato: smooth drives, details have to be added by user, limited HO releases, Kato mostly N scale
Athearn Genesis: highly detailed, improved drive over RTR version, somewhat delicate, premium Athearn.
Intermountain: nice detail, drive/pulling ability hit or miss, long announcement to release dates
Bowser: nice detail, I've heard good things all around.
Walthers/Proto/LifeLike: nice drives, some quirks with some older releases, heavy/good pullers.
Broadway Limited: smooth drive, no non-sound option, a lot of steam options, various past model lines/quality.
MTH: smooth drives, O gauge is their mainstay, details reflect O gauge emphasis.
Rapido: mostly Canadian releases.
Mid-Grade:
Athearn RTR: motors can be smooth or rough, some models have improved details in newer releases.
Athearn Blue Box: historically the hobby's most prolific model. Rugged, simple design. Details can be improved by modeler.
Low End:
Bachmann: many are lower/train set quality, but they have improved offerings. Can fit many budgets & needs.
Tyco: Typical of 70s/80s train set releases, low detail, light, plasticky.
I'm leaving quite a few out, but that's a quick overview. Like I said, any evaluation is very subjective by the poster and
will vary based on personal experience.
Prices have gone up over the years, but so has detail level and variety; mostly the quality of what has been released has as well.
Hope this helps,
Brad