For me, the most intuitive is the DPG-CT, 2300 AD, and MegaTraveller task system It's called the DGP-CT as it was introduced in Digest Group Publications' supplements for Classic Traveller; the MegaTraveller system has some additional nuances, but is the same fundamental math. GDW (makers of CT, MT, and 2300) changed the math a touch by the die swap; the odds are actually better for harder tasks under 2300.
I'll note that I intuited the baseline pretty early in... like the first session I used it.
Note that all three games define skill of 1 as a level employable in trades or as ship crewmen; Doctors and Lawyers are defined as level 3... and certain CT programming tasks need Computers 8... which I've only seen in MT...
The labels are built for the average NPC... CT/MT Stat 7 or 8 (providing +1), skill 1; 2300 stat 10-11 (providing +2), skill 1.
CT and MT, it's 2d6, 2300 is 1d10.
This gives Min/Median/Max of CT/MT 4/9/14, 2300 of 3/8.5/13
Rushing shifts up a difficulty and reduces time; caution shifts down difficulty but doubles time.
Simple 3+ Given the minimum roll, the PC cannot fail.
Routine 7+Usually doable (CT/MT Nat 5+ 5/6 for 83.333% chance; 2300 nat 4+ for 70%); extra time makes it automatic.
Difficult 11+ doable most of the time if you can do it cautiously, risky otherwise; rushing is impossible. (CT/MT nat 9+, `27%; 2300 nat 8+, for 30%)
Formidable 15+ Requires taking extra time or having better than the base 1 rank employable skill and/or attribute... (CT/MT max 14 is below the needed; 2300 max 13 is below the requirement) Note that higher attribute (10+) can hit this rarely (3%); in 2300, this requires professional (as in, Graduate School level) skills or experienced employable skill and high attribute.
Impossible 19+ - the average guy working in field cannot do this, not even by extra time. The MT max allowed die roll gives this a slim chance in both.
I like this kind of analysis; I don't care for the CT/MT using att/5, as it's a 2d6 att; 2300 using att/5 is much better for me, as it's 4d6-4 for atts..., so I up the CT/MT TN's by 1, but use att/3 instead of att/4