Now that I've gave it some thought, I'll rank them from best to least.
1. Classic Concentration, Best of Three. This was the first version of any kind I grew up watching, and it helped me really enjoy the franchise for what it is, even if I couldn't get to see it again between the end of NBC reruns and YouTube uploads, with the exception of a PC game here and there. It truly was CC at its peak, as it allowed a better chance for the second game to be solved without resorting to a speedup, allowed a player who one just one game to still win a decent hall even if s/he didn't win the match. The theming of the sets seemed to be at its peak without being too bizarre like in the second period of the Two Strikes. The 1989 Tournament of Champions used this format, and the Cash Pot later in this run was a nice addition.
2. Original Concentration. There was a reason that if a show held the record for the longest running daytime game show until TPIR passed it in 1987, it had to really be a fun watch. The hosts were great, especially Hugh Downs the various gag prizes were funny, and you still had some interesting twists from time to time like a mystery envelope, a great prizes (sometimes a car) for finding both Wild Cards, and theme shows such as to the Boy/Girl Scouts, celebrity Christmas shows, and of course, tournaments of champions. Just not enough color to land it at the top, and it's always one loss and you're out, even if it had the longest appearance limit of all versions.
3. Narz Concentration, final season. The matching addition to the Double Play round added an extra degree of anticipation prior to the puzzles itself, and the difficulty loosened from the toughness of earlier in the run. Since the expanded Double Play ate up time, it was wise to make the three-call games and extra Wild Card games, and best of all, the four Head Starts meant it was pretty common to get a nice run of prizes. Like what BriGuy said, both contestants got an equal chance. This season also added a bunch of now classic cues from The Price Is Right, and with Johnny Olsen reading the plugs, you could be wondering what show you're really watching.
4. Classic Concentration, "One and Done" format. It was great to see an energetic Alex hosting a new show while having a computerized game board taking the technology to the last level. Unfortunately, the unfairness of the interrupted game providing some losers getting a second chance while others with a full game don't. It's still a good show at this point, but its missing a few things, like Take cards (at least early on) as well as some set additions that came along later.
5. Narz Concentration, Seasons 1-4. The addition of rebuses in color was nice, as was the occasional bonus game if there was extra time after the second game, especially if there was a third puzzle with foreigner currency. That said, the lack of returning champs and especially in seasons 3-4, the difficult Double Play puzzles meant that a car win was very hard to come by, since many puzzles were on the lines of "Charles DeGualle was the President of France during World War II". Some changes toward the end of its fourth season, like the occasional three-call and four Wild Card game, would carry over to its final season.
6. Classic Concentration, Two Strikes. Note that this mostly applies to the final year (1990-91), and not the spring/summer 1988 original format, which basically was similar was a progression from the original era. The reduction of Wild Cards from three to one in the second game also made it more likely to have shorter second games again, the set had excessive greenery/foliage, and it certainly seemed that the show had run its course. Worst of all was the bonus round clock resetting to 35 seconds if a champion was defeated by a challenger, limiting car wins, as the "5 Bonus Car Seconds" usually didn't prove to effective when it actually was won.
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