It depends on what you are doing (i.e. the game itself), the target platforms etc. There is no one size fits all answer; HTML5 is not the best solution for games; it is one of the solutions for games.
For example; if hypothetically speaking; Sega approached you and asked you to create a browser based or mobile version of the latest Sonic Racing game for them; only a fool would consider HTML5. You would choose something like Unity or maybe Flash.
For strictly browser based games you have HTML5, Flash and even Unity (via a web browser plugin). Though the later two do not cater for mobile devices. You can create many simple arcade style games (the types of casual arcade games you see on iTunes) with HTML5; but the more you ramp up the graphics and mechanics of the game the more alternative solutions become more desirable; particularly with 3D games.
There are also instances where HTML5 isn't a viable (or the best) solution because of a particular element of the game being poorly supported by HTML5; such as graphics features, performance, AI/Physics or certain types of multiplayer content.
You also have the issue of whether you are happy for your game code to be open. This may or may not be an issue; if it is an issue then a closed standard such as Flash or Unity becomes more desirable. With more advanced games or ones where some element of security needs to be hidden, your code being closed becomes very important.
You also have to consider turnaround time and other business needs; what will be the most effective route to deployment within a certain timescale and budget; that could influence what the best platform is.
If you are looking at smartphones and tablets as a platform; as native apps, if you are approaching a serious gaming project, nothing (and I mean nothing) trumps Unity; especially when you start to look towards AAA mobile gaming.
The long and short of it is, understanding the technologies; their capabilities, pros & cons; and how your game is affected by them is what will lead you down the correct path for that particular game.