MSPeconomist

DL satisfies criteria 1 and 3. Miles never expire and I've never personally received less than 100% credit for flights but YMMV depending on carriers and routes flown. Also, there are rumors of changes to DL's FF program.

You can transfer miles (not just within your family) but doing so invokes fees that tend to be more than the miles are worth, although this (or purchasing miles) can be useful and worthwhile to top up an account for a particular award that is available now.

Value for miles is notoriously bad with DL. Award tickets exEurope can have high fuel surcharges and other fees too.

If you fly DL within North America, either connecting flights on international tickets or separate domestic tickets, elite status on DL gives you some (small for many routes unless you do 75,000 status miles per year) chance of a free upgrade into FC. You also get free domestic Economy Comfort seats at the time of booking for crediting 50,000 status miles per year. This level also gives enhanced domestic coach award availability and SkyPriority ground handling, with special check in and security lines, plus early boarding. At 25,000 miles, you get a free checked bag, sometimes special lines, and somewhat early boarding, plus even fewer free domestic upgrades into FC and free Economy Comfort at T-24, when OLCI opens. There are reduced fees for international Economy Comfort seats, which have more recline and free hard alcohol in addition to more legroom/increased pitch.