I have a popular game site and because many people know it, I get a lot of blog and forum posts from regular players of my site, for example when they like a new game.

In posts themselves you probably won't find many.

I have a link to an opt-in page in my sig.

Hello Joanne,

You can't get your comment/s at or near the top.

If you want to get notice then do the best thing you can do is to add valuable information within the online community, whether it is blog or forum.
An online community is a place for people to discuss certain ideas and share knowledge that is relevant to a topic. It is not all about promotion.
If you want to promote, there are some forum sites that allows promotion through the use of signature.

Some food for thought...

The uncomfortable truth is that you have to be in the top 5% of posters/commenters to get traffic. Your contributions have to be damn good.

I mean, you only need to look around the Warrior Forum to see a ton of users with massive sig links and posting nothing but 'great post lerned a lot thx'. Seems like a ridiculous waste of time to me; I can't imagine many people would click-thru, let alone open their wallets.

I'm a baseball nut and regularly visit a particular forum. There are quite a few users and a lot of noise, but there is one user who really stands out. He writes well, has a full command of sabermetrics (advanced analysis of baseball stats), and can dominate any argument. I think many of us who use that forum just scroll down threads to look for his input. If that guy had a sig link, you can bet your bottom dollar we'd all have clicked on it at some point.

Likewise, if you wanted to push a site about cupcakes, then it is not enough to do forum posts and blog comments saying 'Great cake, makes my tummy rumble!' No, the kind of contributions that get the click-thrus and build the reputation are those that make competent, skilled suggestions about how to improve the cupcake, or a different way to make a cupcake.

You're into weight loss, right? There was a guy called John Stone who posted a transformation thread on the popular Bodybuilding forum. He updated it every day with exhaustive body and exercise stats, as well as daily photos. That thread went viral, and the guy set up his own website (still going today). It's always the experimental types that get the clicks. As a sidenote, be sure to have your handle as a kind of branding, e.g. 32lbsIn90Days.

On the other hand, if your doing blog commenting, for most blogs, you just need to be first to post.

What you can do is either subscribe the particular thread of the post in form and keep posting valuable comments answers other replies too.. your visibility of the comments increases ultimately you will be on the top.

I doubt if there's any comment that based by ranking like Yahoo! Answers and Youtube comment.
Those top comment are all based by best voted.