E-hentai and nhentai are my two go to sites specifically for hentai. The latter is (mostly) strictly SE Asian manga drawn by hand; the former has that and also permits CG, clanker slop, and western comics, too. Mangadex also has quite a lot and tons of family friendly ones, too.

And there are GreaseMonkey scripts (or downloader repos) for them that make the offline backup process easier.

If you like manhwa to read vertically on your phone, then manhwaus (was manhwascan) is good.

All the above are aggregator sites. There are others, and also specific artist group (eg. writer+artist) or scanlation group sites around, too.