Everything requires Runtimes of a sort.
Assembler has runtimes built into the bios/CPU of the chipset you're programming against. Its for this reason that with similar chipsets you can default to more general code and make a file that will work on most PCs. If you start optimizing things it won't work on all PCs.
C/C++ generally has the runtimes built into and distributed with windows. I'm not sure about Linux or OSX.
VB6 requires runtimes, but they're already installed on most Windows (Post 98 I think) machines.
.Net is in a similar boat with much larger runtimes. Everything past XPsp1 has the framework.
Java isn't included in any OS's that I know of and like .Net a lot of people are against installing it, partially because of how intrusive it is and partially because it's Java (don't ask me).
Flash is the easiest to install of the multi-OS programming languages and easiest to work with graphics. You can create executables for specific OSs like Mac and Windows and the 500k runtime files are easy to come by and most people have them. Just like with .Net & Java a vocal minority has a vandetta against it and loudly whines about hating it. The only way Flash has ever annoyed me is how it's used for banner ads (replacing animated GIFs) and when people misuse it to make redundant "intros" and animation/graphic/sound heavy web sites.
I'd say Javascript would be your best bet as everyone has a web browser, though then you have to play "browser wars" as each displays and utilizes JavaScript differently. Then there is the vocal minority with a valid claim about turning Javascript off for security/privacy reasons.
I'd say in order of accessability you have: C/C++, Flash, Javascript, VB6, Java, .Net
Flash is pretty much program and forget, C/C++ and Javascript you have to really code to your user(s)