I decided that the best way to measure that was by the margin a candidate exceeded his party's Presidential nominee in competitive districts. So a candidate who gets 59% in a district where his party's nominee got 51% will be more "valuable" than one who got 52% in a 51% district.
It's arguable that the party can't afford for either to retire, as both held tough districts, but the 52% candidate might fall in the next wave while the 59% candidate won't. You also don't need independent expenditures to help the candidate who got 59% win.
I'm including those candidates in D/R +3.5 or less districts who exceeded the Presidential nominee, in 2 party numbers, by 6%. This list has 20 Republicans and 7 Democrats. Overall, Republicans hold 34 such districts and Democrats hold 28. Note: If I extend it to 5%, I add 3 more Republicans and no Democrats.
If you're a Republican you can take that we have stronger incumbents in competitive districts. If you're a Democrat you can point to big opportunity with retirements and that your own retirements won't increase the number of competitive districts. Of course that means that NY-1 will always be competitive but NY-2 will only be if Peter King retires.