I know the thread is a bit old but I didn't see any good reason for starting a new one instead of adding to this one. Watching 実況プレイ vids has helped me immensely with learning Japanese, has made me laugh a lot and been all around awesome. So, here's some of the best ones I've seen:
せがれいじり: this is definitely the oddest Enix game I've ever seen. it's just totally nonsensical and ballsy and rad, more games should dare to do things like what this one does. it relies heavily on wordbased jokes, so from a learning perspective it can be very good for learning how to joke in Japanese and how you can play around with the language itself.
http://www.nicovideo.jp/watch/sm10100877
かまいたちの夜2: the game is a quite humorous interactive murder story and heavily textbased (it's a direct follow-up of the first game but it explains most of what happened in that one. I haven't played/seen the first game played but I had no big problems understanding the story). the player is what really makes this special as she reads most of the text out loud and does different voices for each character (sometimes while she reads she says something else from what is written to make a joke, that threw me off at first). she's very funny, the game is long and it's overall an ideal 実況 for learning Japanese. seriously, this is of such high quality that it would be worth paying for as learning material. she also has videos from playing through the third so in total there's probably 15+ hours of text with audio + helpful pictures to go with it. I'd recommend turning off the comments for most of the time as they tend to be in the way.
http://www.nicovideo.jp/watch/sm10394747
パチパラ13: a pachinko simulator. not even kidding. only, it has loads of content and there's a storyline (which always lets the main character save the day by playing pachinko in an utterly MANLY way), very funny dialogue choices, a big big city with shops, different apartments to buy, a driving test, etc etc. when I checked some reviews I saw a description that fits it quite well, "無駄に神ゲー”. the player is very good at rolling with it and makes good use of the freedom the game lets you have instead of just picking dull, "normal" choices.
http://www.nicovideo.jp/watch/sm13630689
Resident Evil 5: this might not sound so interesting. however, the main player is playing through the game mostly just following his co-player around while commenting on everything like he was a soccer game commentator on steroids and misunderstands everything on purpose. it's frantic, funny and very different as he manages to change the whole appeal of the game and turn the whole thing into a special show. check out the first episode and you'll hopefully understand what I mean. he speaks VERY fast so don't sweat it if you don't understand even half of what he says
http://www.nicovideo.jp/watch/sm9808307
Gundam for PS1: this game is terrible and definitely worthy of the "so bad it's good" achievement. I don't want to spoil anything but if you have any idea about what Gundam should be, and probably even if you don't, you will laugh. The player himself is very funny too and if you feel you want more of this (he doesn't play the Gundam game anymore after this one video), follow the second link to find his full playthrough of a similar game made by the same company
http://www.nicovideo.jp/watch/sm5487778
http://www.nicovideo.jp/watch/sm4600152
Zelda - Ocarina of Time: this is a cancelled playthrough consisting of only four videos and it's a shame the player didn't continue this project. like the title (もしも自分がゼルダの主人公だったら) implies, the player plays/talks as though he himself was the character/Link and does it very well. this is also where I learned ごまだれー!
http://www.nicovideo.jp/watch/sm6910217
Lastly, this one probably doesn't do much in helping you learn Japanese but it's ridiculously hilarious. There's a guy who calls himself Pikachu/P who is playing through the 2D mario games in quadruple speed (look up his videos if you're interested, myself I got a bit bored by them) and this is what happens when one of his videos is slowed down to normal speed - making his speech four times slower than normal
http://www.nicovideo.jp/watch/sm5745173
Thanks for the recommendations in this thread!
Edit: Okay so this isn't a real playthrough and there's no accompanying commentary but it's the most hilarious thing I've probably seen in years. Someone give this guy a job somewhere because AW YEAH. It's Zelda: OoT again but it's been changed into something entirely different and it's just awesome. Deeply recommended to anyone who has played OoT before.
http://www.nicovideo.jp/watch/sm8227436
Edit2: Here's a playthrough of 探偵倶楽部 (Tantei Kurabu) 2, a detective game classic that really knows how to build up suspense, has a bunch of well-written characters as well as atmospheric ambient music and also gives you a lot of chances to think. If you have any interest in mystery stories, you'll love this. The players are an incredibly entertaining duo, once they pick up steam in the fourth episode or so they also start doing a lot of speculation/mystery solving on their own which adds so much to the game. Moreover, they are generous enough to read all text out loud which is, as always, super nice for us language learners. In the earlier episodes they have some problems with mic noise, and yes it's annoying, but please stick with this 実況 because the noise problems are solved later and it is an extremely gratifying experience to follow them through the game. Even the comments in the videos are good, it's nice to watch something with only comments from a few users instead of the usual scrolling wall of text. Highly recommended.
http://www.nicovideo.jp/watch/sm14353717
Edited: 2011-10-23, 6:11 pm