My stance on gameplay in relation to commentaries is the social element is much more appealing now than it was years ago.
YouTube has become the domain for aspiring film producers and commentators, and it is easier to produce content and appeal to the community now than during the early years of YouTube history. There has been a major shift from videomaking as a hobby to the opening and opportunity for others to treat videomaking as a profession, as early YouTubers paved the way and defined the formula to making successfully appreciable content.
I personally take commentaries casually.
I find them interesting because it allows one to relate to others through their experience of the game. The social element of interacting with a community notwithstanding gameplay allows people of all walks of life to relate to games, and I find this fascinating.
Commentary doesn't just add fluff to the game. It opens up narrative from a new perspective, and allows others to discover new ways of playing and using divergent thinking.
Favorite series so far:
Prior to Terraria's release, PaperBatVG had contacted Redigit and Blue (Main devs) for early access to Alpha of the product. He made an extensive playthrough spanning 32 episodes and even got a chance to play with and interview Blue based on feedback and questions from the community.
His LP was comprehensive and described the intricacies of the game.
He displayed most of the available features and offered perspective on how the genre relates and differs to Minecraft to clear the air of speculation, as well as other games both through his open opinions and through gameplay.
Although he did have some hiccups during the commentary such as missing small details, such as availability of updates (When the game updated to include money loss as a penalty for death, he died and didn't realize he had lost 1 platinum currency, the most valuable in the game until episodes later. Silly I know, but I forgive him.), but it is excusable because he produced enjoyable yet informative content.
Definitely learned from his LP, and I've considered getting Terraria now.
I understand Yogscast also was streaming LPs of Terraria, but PBat was my source for content.
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Secondly, ONLYUSEmeBLADE is in the midst of streaming a GTA:SA LP.
I appreciate his commentary because he takes a somewhat lax approach to the game in regards to collectibles, and focuses more on the story.
As a regular to the YouTube community, he is very open to feedback from comments from fans and viewers in his videos, such as useful armor locations, different methods others may suggest at approaching problems in a constructive manner, and addressing general complaints such as his commentary blocking out the voice acting during cutscenes, which he utilized the subtitle feature and implemented it within an episode.
He leaves content such as grinding Muscle stat, biking skill, etc. out of the LP and leaves room for the fluffy stuff. I'm enjoying his amusing quips and interjections about aspects of life, double standards, and behaviors during the series, which also signifies that he is having fun doing the LP as he immerses himself into the game.
It's mostly nostalgia that grabs me to his playthrough of GTA SA as I sunk many hours into the game. Also, it's fun to see a playthrough of the game from another perspective as I've never actually seen an LP for GTA SA yet.
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Lastly, AzuriteReaction has what I believe to be some of the funniest commentaries around. He's got a colorful personality, he's a cool guy overall, and his act is like penguinz0 mixed with the Rage Guy meme, but with many, many references to pop culture.
His delivery of a consistent amount of content is superb, and he communicates with his fans and the Youtube community on a regular basis, as well as having a blog to keep schedule.
Plus, his recent videos are mostly games I have never even heard of, some being indie games, some flash games, and others are major titles such as his Rock Band series. It really opened me up to the strange, quirky and wonderful titles available, and this mix of content is really healthy for his channel.
Small EDIT: If I had a favorite, it'd be SSoHPK. He has many quality videos and uploads videos often, like a BOSS.