Really you have to take both into account a little bit but worry more about the actual page than the parent site.

Also look at how well the top 10 pages are optimized. Do they all have exact matches of your keyword in the title? What are the links for all the top 10?

If you haven't already google 'SEO Quake' and download it, best free chrome/firefox plugin going for this sort of stuff.

Check your keyword with the keyword tool, make sure to click exact match, if it's 10,000+ and every result in the top 10 is optimized for your keyword, you might want to try something else if you are just starting out.

Cheers

I think the back link posted in the first of any topic is indexed by Google easily so the first back link of any topic the link that works really well .....

Not much, because it's a quantitative approach, not a qualitative one, so you can't reliably learn much from it at all. What matters is what the backlinks are. If most of them are from context-relevant, authority sites, then even if there are fewer than 100 of them, that'll be a very, very difficult target to beat. Try to stop thinking about "how many" and start thinking about "how good". It's a very good "general principle" for the whole of internet marketing, actually.

Backlinks work on a "page basis", not on a "site basis".

One more thing to add to Alexa's points is to check what the anchor link text is. I have seen sites with 100's of backlinks but only 4-5 of them were even aimed at a keyword I was aiming for. All the other backlinks were aimed at other terms.

Another thing you may want to know, although yahoo show the majority of backlinks a site has. Don't take that number as the 'absolute' amount, because they don't show every backlink a site has.

And the backlinks missing may be stronger than expected, whilst the backlinks visible seem non-challenging.