First Edition changed the world of RPGs. 8o)
Second Edition changed how magic use was accounted for by introducing Magic Points instead of "Temporary Pow Loss".
Third Edition split the rules into Player and Keeper books but was otherwise much the same as the previous two boxed sets. The hardback book version of 3rd edition combined the rulebooks and the sourcebook into one volume and added the contents of the First Cthulhu Companion (six extra scenarios including one for one player plus a keeper) and six colour plates.
Fourth edition added new colour plates and was the first paperback book version. The Cthulhu Companion was not included. Scenarios were changed.
Fifth edition (I think, I never played Fourth so my info on that is from a quick skim) combined Oratory and Debate into Persuade and changed skill point allocations from EDU x 10, INT x 5 to EDU x 15, INT x 10 (if I remember correctly, I don't have the books to hand). Chase rules were introduced.
Fifth and a half edition (5.6) reformatted the rules, added sourcebook style material, removed colour plates and replaced them with new B&W artwork.
6th edition lost some of the B&W art, added the "ancient tome" graphic design elements to the page edges, scrambled the rules and the Bestiary so that you can't find stuff in-game without stopping, and made the simplest RPG in the world almost as hard to pick up for a beginner as GURPS. Containing self-contradictory information and advice, this version was first seen as the 20th Anniversary edition where it was printed in pale red ink on soft matte paper, and has been reprinted unchanged but for the endpapers as the 25th and 30th Anniversary editions.
I haven't joined in the lunacy over at Kickstarter concerning the seventh edition, which seems to be incorporating ideas already available under alternative game systems set in the Cosmic Horror/Cthulhu Mythos universe, notable Trail of Cthulhu and Realms of Cthulhu. One wag opined that as an antiquarian he knew for a fact that one could obtain real ancient documents for half the price of the Red Leather and Gold "more money than sense" edition of Ed7 ($3K buy-in). I'm running games now, not collecting for its own sake, and everyone wanted to use 5th edition.
If I had to point to one edition and say "best value for money" it would be the third edition hardback book version. The last version that had its eyes on the prize from an ease of assimilation and playability standpoint in my opinion was version 5.1 (and nothing says that someone took their eyes off the road like the drive to publish under document revision numbers instead of market-friendly Editions).
The paperback 5th edition can be gotten for less than 10 dollars from Amazon. Third edition is a bit harder to come by, but is, I feel, a better product.
(All that was from memories of my own collection currently sitting 40 miles away. Please forgive inaccurate information which is due to old brain and the fact that I obtained fourth and fifth and a half editions later than the others, as collector's items rather than as game aids. My own gaming knowledge comes from editions 1,2,3,5 and 6.)
The later editions (fifth onward) included some basic stuff on Current Day and Gaslight milieus, price lists and suchlike. I honestly feel this was a mistake because moving the time period is something I feel needs a dedicated sourcebook to do any sort of justice to the material, but I'm pretty much alone in that.