There is no "best". Depends on your needs and other factors.
What's your source (mini-DV, mini-DVD, still camera video,?) Do you have a budget? What do you want to do with it (add simple titles, add a lot of effects,?)
For starters unless you want to export your video source directly to the web (e.g. no editing) you need to seperate the web aspect from your editing software, some editors can export as web ready video but this is more of an add-on feature and is not necessary to get it to the web.
You need to consider the things I've listed and pick an appropriate video editing application. Conversion to web video is secondary.
Having said that for all around general use Ulead Video Studio is quite good and doesn't cost a fortune. Very easy for beginners to learn. Video like most things is not a sit down and churn out "The Godfather" overnight type of proposition. You can achieve a lot with simple programs like the one I mentioned but the "best" professional programs have a steep learning curve and price tag to match. Additionally people that do this a lot or professionally have an arsenal of tools they use, it's not just limited to one program.
AS far as making it go viral wish I knew the answer to that, and if I did I certainly wouldn't tell you. :P