(I'm sorry this is a small post, I just got done making a HUGE IN DETAIL post, when Firefox shut down on me, deleteing what I had written. I'll be making a smaller one instead. QQ)
Freeman's mind is clearly the best, however not because of it being the first, but the quality is just better of the rest, allow me to explain.
Freeman is what we never expected, he is a egotistical smartass with paranoiac and sociopathic tendencies. Freeman openly admits how he hated his job before, and now still does while also commenting on the stupidity of the whole world around him. Its the mix of all of these eccentric personality traits that make him a truly original and memorable character and sets him a part from the rest.
Barney from Barney's Mind is sadly however just a complete homage to Freeman. He shares several traits and just copies many things from its brother series to be called an original creation. Its only near the end where he becomes his own voice and actually becomes notable. It isn't a bad series, but I can't laugh at all the jokes I've already heard before come out of Freeman's mouth.
Shephard however from Shephard's Mind is a truly original character. Clearly the second place winner here. Shephard is his own voice, and never feels like a rehash of Freeman. Shephard is less egotistical, less eccentric, and also more professional then Freeman and Barney. He is also more original because he is a marine which gives him such traits as honor, devotion, and loyalty. However he is also susceptible to being paranoid and sarcastic like Freeman and Barney. Also it should be noted that he has an original character trait of being somewhat cowardly compared to the first two on top of all of this. He makes friends with everyone he can, and even talks to his pet alien things to help pass the time. While he like the others pokes fun at Black Mesa, he does so in his own unique way. All of these traits make Shephard a clear standout from the rest.
Chell from Chell's Mind is a standout in her own way. She does not like cursing much, tries to stay calm, and tries very little to make sense of the situation after the first episode or two. She is clearly insane, but is not aware of her own lunacy and continues on as normal. She treats her portal gun as a person and regularly attempts to have discussions with it. She freaks out at a radio for playing a song she deems as "trying to use psychological torture on me!" and then destroys it. She is clearly a schizophrenic individual with little disregard for anything. While I wouldn't go as far as saying she is a funny individual, I do have fun watching her in her test chambers like a lab rat working her way around.
Felix from Felix's Mind is original in the sense he is unlike the first four. Its just a guy having a really shitty day while being confused as Freeman. He is definitely eccentric, but also much more sane I would say then the rest. He seems like he is on par with Shephard in terms of sanity, but on par with Barney in terms of ready for combat. He is overall kinda enjoyable, and worth trying out.
Parker from Parker's Mind is also just like Felix, however a completely different game then the Half-Life series entirely. However it should be noted he is an underachiever who rises to be a bit of a badass, so its one character trait he has over everyone else. Enjoyable, and worth trying out.
Those are all the ones I've watched more then a few episodes of, so those are the only ones I feel qualified to give my opinion on. It should be noted that all of the ones I have mentioned are good online friends, all of them have talked over Skype and talk with their fans together, some of them even did videos together. All of them are nice people who from my impression talk often online.
http://roosterteeth.com/groups/profile.php?id=8111 (Its a great group I'm a part of on Rooster Teeth's website. It has put together Skype chats in the past with the creators and answered questions if I recall.)
So yeah, thats my thoughts on them all, I'm a huge machinima fan overall and the "Mind" fad is one I've followed closely.