Are you replacing just the head unit or upgrading the entire system and adding things like amps, capacitors, and subwoofers? For head units i looked for preamp outputs i could boost with my own amplifier and picked one that had simpler controls and without all the flashing lights. I want less distractions while driving. Since they all include eqs just about i look for the eq features i want such as individual controls over the frequencies rather than presets for certain kinds of music.
The brands have changed quite a bit in quality. I used to lean toward Alpine but they didnt seem as good the last time i was shopping. I still like to go to a stereo place where i can listen to different components but those are getting fewer and farther between. For speakers i have had good luck with JBL