Innerleithen’s waymarked trails are mostly pretty straightforward, better than most for a first step into official downhill.
I say official, because there’s absolutely no difference between downhill and just riding your bike, if you took the 4 main inners tracks and piled them on top of each other and stuck them up an alp then folks would happily ride them on holiday, but in the UK they are downhill and therefore Not For Us. But then they throw them into an enduro race and loads of folks who wouldn’t do downhill go and ride them and have a blast.
Only trouble with innerleithen is that there’s no one “easiest” dh. Not counting Make or Brake anyway 😉 Oh and also there are trails everywhere so it’s easy to start out on a main, simple line then end up doing something else entirely. Gold Run is probably best to start on but goes relatively steep at the bottom (and the bottom bit is currently all kinds of loose). So you end up recommending something elaborate like doing upper and middle gold run then switching over to matador lower, or just doing matador without the drops.
Another option that might sound silly, is fort william. Start out on the nevis red to get your head adjusted, if you’re comfortable and up to speed on that then cross over to the world cup route. It’s actually not as hard as you’d think, especially if you stop for rests! It’s not easy, but any capable rider on a decent bike should be able to have a decent stab at it. Possibly using some cheaty lines or walking short sections the first time.
If I can ride it on a hardtail on xc tyres, despite being fundamentally a knobber, then how hard can it be? 😉