I have both the 35 & 40 Nokton, and they are usable choices on the M8 except that there are no 40mm framelines. Agree that 35 is a useful "normal", while 50mm is handy as longish (~66mm) normal.
I find 75mm "too long" for me, so 50 is the longest I go. A 28mm is a nice "wide normal" at around 37mm equivalent, then 24/25 is very useful if you like the 32/33mm equivalent. Both the 24 and 28 make for good walk-around street lenses. And 24 is as wide as the framelines go... So while 21mm is a great choice for the 28mm equivalent, it calls for an external 28mm viewfinder. That's fine with me but some object.
One nice thing about the M8 "crop sensor" is that it makes these very wide lenses (15, 18, 21, 24) more useful, and further, we don't get the edge/corner effects that make these lenses troublesome on the M9, 240 etc.
One other thing to consider is that the M8 may have differently "optimistic" framelines depending on whether it's the original, or if it's an M8.2 or M8 with the frameline "upgrade". Original framelines are most accurate at something less than 1m distance, with a goodly bit outside the framelines captured at infinity. The frameline upgrade and M8.2 moved that accuracy point out to 2m for a bit of compromise.
The 40mm lenses typically bring up the 50mm framelines unless they've been filed down, as some users do, resulting in the 35mm frames showing instead. Some say the original M8 frames are good with the 40mm lens at moderate distances. I have not tried that, because... with other focal lengths one needs to remember the "overage" so usage becomes inconsistent among lenses.
Have fun shopping!
And, I'd suggest choosing a focal length first, then shop for a specific lens of that focal length.