I would put both 14.1 and one pocket in my top 3 games (american rotation is up there as well), but the order of those top 3 varies depending on exactly what the criteria is. For player interaction, I think one pocket is pretty hard to beat, so I find it the most interesting of the 3 to watch or play if I'm in the mood for a head-to-head tactical battle. 14.1 gets the edge as a solo practice game or a showcase of skill and concentration. American rotation is nice because it brings the challenge of running balls in order, but with rules to mitigate some of the luck involved in other rotation games (especially the break).
As for one game being more challenging than another, it really depends what your strength and weaknesses are. If you are prone to frequent lapses of technique or concentration or have trouble working the cue ball in tight spaces, 14.1 is going to be tough. If you have trouble playing longer position routes and kicking, american rotation is going to seem difficult. If you have trouble anticipating your opponent's move and controlling the cue ball and object ball speed at the same time, one pocket is a challenge.
Fortunately, all 3 of these games require the same equipment to play, so I'm only limited by what my opponents are interested in playing, which is why I play more 8-ball and 9-ball than anything else.
Matt, one thing I do like about 1pocket is that, unless you're playing a monster, you get to the table often. At 14.1 you can be spending very long periods in the chair and that, perhaps more than anything else, led to its decrease in popularity. People want to shoot and 1pocket generally affords that.
Lou Figueroa