From what I know, there are companies (for example Blizzard Ent, Riot Games, indie devs...) that let monetize let's play videos through partnership programs on YouTube, Twitch and similar pages. It really depend on which game you want to use.

How would I know what games are "usable", then?

To be honest, I don't know. I only mentioned that people are allowed to monetize some of them and wanted to point out that not everything is in the grey zone. I watch some casters/commentators/let's players on YouTube, and they're mentioning from time to time, that they have to be careful choosing games for their series, because they also don't know what they can use and what may cause troubles. I guess most of the times it's not specified in policies.

Damn, this keeps bugging me...

So I guess it's not "legal", but will I get in trouble if I make a few TOP-5 lists that has clips from a game, music or both?

erm, opinions?

YouTube and the like are generally in the United States so operate under the fair use concept (rather than the laws of the uploader's own country). Fair use doesn't specify exact percentages of what's fair and what isn't, but generally speaking both the hosting sites and the copyright holders are OK with it as long as you're using a reasonable portion of the copyrighted content, not the whole thing.

If you want better replies than the ones you already got, then I think you'll have to ask a lawyer - and not any lawyer, but one who's in your country and who specializes in international copyright law.

That's not meant to sound snide (I'm a bit afraid that it does sound that way, but I don't mean to be). It's just that the copyright situation is so complicated that no better answers can be given by laymen.

My personal opinion: Just do it, take your own conscience as a guideline, and don't be too sad if Youtube suddenly removes your clips. I don't think the risk for that is very high, but it does exist. You may want to check if you're making yourself a target for cease and desist letters which involve costs, but I think that this abomination is a specialty of the German law.

if you use any videos of or soundtracks , be sure to credit them before your video ends that should remove the illegal use, for example sonic is a trademark of sega bla bla , image provided by yada yada etc,

watch some of the angry joe videos and see how it ends

So if I make a Top5 -list of most memorable video game music, add a short clip of the song and have myself talking in before the clip and later on top of that, making it a "commentary" of sorts, what's the worst that can happen?

Can I get that infamous copyright strike because of that?

The kind of use you describe has an incredibly strong claim to fair-use (as it uses only a small portion of the original works, is transformative, is non-commercial, and has no reasonable negative impact on the value of the original work). Of course, even with such a strong fair-use claim this doesn't mean that one of the copyright holders won't be a complete ass and file a DMCA takedown, or that YouTube's automatic filters won't recognize it as containing portions of copyrighted works and either slap ads on it or automatically take it down (their filters give no consideration to fair use).

Worst that can happen is that your video gets taken down and the YouTube account you used to post it gets suspended.