I have to agree with Shabbir. Although PR is still a factor, it seems that its importance is diminishing all the time. Neither Penguin nor Panda respect PR and sites have been hit regardless of it. I have only been involved in SEO for less than two years but the changes (as far as Google is concerned) have been immense during that time.
In every other SE my keywords still rank very well, but in Google I am struggling and like it or not they are still the main traffic providers in terms of organic searches. If I go to duckduckgo I am number one for multiple keywords but I don't get any traffic from it so it is pretty meaningless.
I do get traffic from Bing and Yahoo but although it is significant, the fact remains that in my market, the majority of users use Google.
Although you do not specify in your question which search engine it is that you want to improve your keyword ranking for, but I think we can assume it is Google.
Much depends on their competitiveness and how well established your competition is. You may need to look at some alternative keywords.