Depending who you ask they all are. The 3.9 was the mainstay of the RRC and later Disco, some had problems with the water jacket and failed but TBH the age they are now they're either fine or bust long ago.
The 4.2 is a 3.9 with the Iceberg crank in it, all the sensors, injector etc are the same but you'll need the 4.2 (LSE or Autobiography) ECU for it and they still command a lot of cash second hand, alomost worth fitting a Tornado chip. Lots of LSE engines have done starship mileage too and quite a few suffered "porous block". Again if it's still going it's probably ok 'till it just wears out.
4.6 would be nice, the crank is stronger, the heads better and the best blocks were selected for the 4.6 to prevent block failure, although I've read reviews that claim the HP24 gearbox held the 4.6 at low revs in high gears for economy which causes uneven heating of the liner, leading to block failure.
Anyhoo, to fit a 4.6 to an older car you need to do a bit of work. The front cover will fit but you need a spacer on the crank and a new camshaft to drive the dizzy, or fit the (rare) interim V8 front end from 300 series RRC or Disco. These have the better crank driven oil pump and last that bit better because of it, though a new pump means a new front cover=big bucks.
It's more a matter of finding a good engine that is worth the money. As a guide the 3.9 was 185BHP, the 4.2 199BHP and the 4.6, well it was never fitted with the 14CUX EFi system as standard.
