To max out your AP without wasting too much time: Do every difficulty of nighthold you can, get a 10+ M+ done, do a heroic daily random queue, do each day emissary and every broken shore WQ for turn ins, then do AP world quests. If done with all that i've heard ppl say regular mythic dungeons and even heroic dungeons with a group that can just plow through them. There's not a real good "infinite" way though because you'll run into daily lockouts on heroic dungeons if you do enough and they really nerfed M+ end chest ap.
But really if you have time for all of that shit each week and all the WQ and heroic dungeons each day that's pretty ridiculous.
Someone on reddit posted a thread about ap/time spent on new m+ dungeons and it was split up like this:
Short = maw
med = cos, dht, cathedral, vault, both kara, eye, nelth, brh (lolwut)
long = arcway, hov
the only ones that had decent ap per time invested seemed to be court (since it can be done so quickly and gives medium) and arcway (since its considered long but really doesn't take all that long). possibly hov too with the changes made to it, because they removed some trash and stuff.
Basically the take-away though was don't do mythic+ for just ap. If you want gear from somewhere or are working up a key for your 10 for the week, or just enjoy doing dungeons with friends, sure, but it's not a good way to farm AP anymore.