For ebooks, an e-ink display, preferably one with a faint backlight (e.g. Kindle Paperwhite). The problem is that the "white" on an e-ink display is still fairly grey. The backlight helps brighten this up so it looks more like the white from a sheet of paper. Don't think they make phones with e-ink displays. But an ebook-reader is cheap and small enough you can afford to carry it around as a secondary device while you travel. It'll also help preserve the battery life on your phone.
Comics (and magazines) are different in that many of them are color. For those, I prefer OLED/AMOLED simply because of the better viewing angles. IPS has good viewing angles too, but their weakness in my experience is that while the colors don't shift...