Obviously it is difficult to generalize as orphan miles that may be useless to one person may be quite useful to another to top off a high value award. Similarly a trade may be quite beneficial to someone with a surplus of air miles but no hotel points. And generally speaking, a trade that permits redemption for high value awards, i.e. business class seats or 6-night resort stays will be more beneficial than domestic coach or 1 night stays.

That being said, off the top of my head, and with the usual caveats, I would think the following rates would be competitive for a trade:

Airline miles - 1:1 for UA/AA/DL/NW, perhaps less for CO nonepass miles which are hard to redeem at saver levels (but easy to launder via Amtrak at 1:1)

Hotel points - 1:1 for Hilton-Marriott-Hyatt, 2:1 for Hilton/Marriott-SPG or IC-Hilton/Marriott.

Airlines to Hotel: 1:2 for UA/AA/DL/CO/NW to Hilton/Marriott/Hyatt, 1:1 to SPG (SPG points can be converted backwards at this rate, or even better as 20K SPG = 25K airline miles).

WN: $15 per credit, or $250 per RR roundtrip ticket (maybe a little generous these days given the limited RR inventory for popular dates/times/flights). Converting to points, that would be 1N at an expensive hotel (i.e. SPG Cat 4, Cat 5-7 at Hilton/Marriott) or maybe 15-25k airline miles.