I've been lucky enough to watch hockey in many countries (and others on TV).
I'd rank the AHL as second behind the NHL and just a half step above the top European leagues.
I'd then rank the KHL*, Swedish SHL, Finninsh Liiga as the top tier outside of north America.
Then I'd rank Swiss, Czech at the next level, probably with the German DEL slotting in around there also.
Some of the second tier leagues might come into play around this level - the Swedish Allsvenskan. I would also slot the ECHL in around here as competitive with teams from this and the next tier coming:
Then you get into a tier with the Alps Hockey League (Austria, Italy, Slovenia), French league, Polish league, Slovak league (which has some Hungarian teams), Danish league, Norwegian league, German DEL2, British league etc.
Now that's not to say for example the top Swiss team isn't better than a top Swedish team, but just on a sort of overall depth of league.
As to your question on the Marlies v a KHL team, I think it would be pretty even but I think an AHL team would shade it in a series.