This is a rough draft, but I think it's about time we talk about this, before we start using Vs2/VsX in situations where it really isn't appropriate.
Adjusted points (points adjusted to overall league scoring)
What it does:
The goal of hockey is to win games. In a higher scoring environment, you need to score more goals (on average) to win, therefore every goal scored is worth less. Basic "adjusted points" simply adjust a player's point total based on the average number of goals scored each season.
Mathematically elegant. In a mathematical sense, the formula strictly determines how much each goal is worth and adjusts a player's scoring total
Adjusted points take into account overall league scoring, therefore they are the best method to determine the offensive value of the average player.
Adjusted points perfectly account for the total amount of scoring, but not the distribution of scoring. It has been mathematically proven that in some eras, first line players score a higher or lower percentage of the league's offense. Adjusted points do not take this into account and will therefore underrate star players from certain eras (in particular the 1980s) and overrate star players from other eras.
Hockey-reference's version of the stat introduces questionable variables that cause results that don't pass the smell test. Their numbers are pretty good post-expansion (where the goals to assists ratio and the size of rosters are relatively constant), but should not be used for pre-expansion players.
Unfortunately, nobody else has really calculated adjusted points for pre-expansion players.
Adjusted points are a good method of comparing the offense of average (non first line) post-expansion players. For ATD purposes where post-expansion non first-liners will rarely see power play time you are better off using adjusted even strength scoring. This would be a very valuable tool if someone would take the time to come up with an appropriate formula that applies to pre-expansion.
Adjusted even strength scoring
What it does:
Same thing as adjusted points, but for even strength scoring only. Adjusts a player's point totals to the overall league scoring environment
Adjusted points are most useful for "average" players, and such players not only would probably not be used on an ATD power player, but also would probably have received inconsistent powerplay time in real life. Therefore, adjusted even strength scoring will probably be more useful around here than overall adjusted scoring, when it is available.
Only available for post expansion players
The best method for comparing the offensive value of post-expansion players who rarely received first PP time.
Percentage methods (Vs2, VsX)
What it does:
Attempts to solve the problem presented by adjusted stats - the problem that first line or star players score a disproportionate amount of league offense during different eras. It does this by pinning all comparisons of offense to star players - the second best scoring in the league in every year (or in the case of VsX, the 2nd best scoring except when we have reason to modify the standard).
Advantage over adjusted stats: By selecting one of the league's top scorers as the standard and comparing all other players to him, the percentage method does what it is designed to do - track the offense of the league's best players.
Advantage over top 10 or top 20 finishes: Assuming the standard selected is reasonable, Vs2 or VsX is much less affected by the level competition. If a player is 90% as good as the standard, it doesn't matter how may or how few other players performed as well that given year.
This appears to be the best method at taking into account the influx of European talent in the early 1990s.
Of all the methods, this one seems to be the most affected by league size. It is mathematically proven that the number of players meeting certain benchmarks increased dramatically when the league expanded in 1967.
The probable reason: the Original 6 era had only 18 spots available for first liners, and players usually played the full power play. Any player who didn't get one of these spots would score significantly lower by this method than he would in a larger league where he got those offensively opportunities.
This method should only be used for players who spent their entire primes as scoring line players who received first unit power play time. Caution needs to be used when applying the method to second tier stars from the Original 6 era, guys who would absolutely be good enough to play on the first unit in an expanded league, but who didn't because of the smaller league. How do you figure out when you have one of those guys? If he didn't regularly finish in the top 10-15 in in scoring, he probably didn't have one of the prime 18 spots in the league.
The best method we have for comparing the offensive value of scoring stars across the eras.
Also a great method for comparing the offense of other scoring line post-expansion players with each other, even if they weren't necessarily stars.
Pre-expansion players who did not spend their entire primes in offensive roles should NOT be compared using this method.
Top 10/Top 20 points
What it does:
A list of the top 10 or top 20 scorers in the league in any given year. Duh.
The least affected by league size of all the methods. A 10th place finish is a 10th place finish no matter the size of the league.
Ease of use: Top 10 finishes are easily available on hockey-reference and top 20 finishes are easy to calculate.
East of use 2: When attempting a crude comparison with a player who didn't play in the NHL, Top 10/20 finishes are easier to fudge/estimate.
The most affected by the talent pool. A 10th place finish can be vastly more impressive in a deeper talent pool. This is where the percentage method is a major improvement.
Arbitrary cutoffs: Top 10 finishes make a 10th place finish look much better than an 11th. Top 20 finishes make a 20th place finish look much better than a 21st.
An adequate "default" method if no better method is applicable, so long as you then make note of the talent pool.
Unfortunately, until someone comes up with a form of "adjusted points" that passes muster for pre-expansion players, this is probably still the best method for comparing the offense of second tier pre-expansion players.