Hi-tech C51 - I know that compiles for PICs, not sure about 8051
8051 Metalink assembler - It's for the 8051
I only have experience with PICs so I can't comment on the 8051 but as far as which language is the best well, that's kind of a matter of opinion. I use C and assembly and you can learn either one first. I have MikroC but Hi-tech is an OK compiler too. I haven't played with Multisim before but my understanding of it is that it is a circuit simulation software package. My point is, even if you're simulating a microcontroller circuit, you probably won't develop your code
in Multisim. You will probably write the code in a standalone IDE, compile it, and then just load the .hex file into Multisim to see it emulated. I could be wrong but that's my guess.
Long story short, there are no real right or wrong answers to your question. You're asking for opinions. What you're going to have to do it just pick one and jump in. As you work through a few tutorials the pieces will begin to fall into place and then eventually you'll build your own preferences.
Also, you might get more response to this thread if you posted it in the
micro-controller forum. You can either ask a mod to move it for you or repost in there.