Can anyone recommend a good SEO plug-in for WordPress? I have tried a couple but I have had marginal results. Any advice would be appreciated.

Yup, I use the "all-in-one" seo pack too... easy to use.
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all in one SEO pack is the best for wordpress

all in one SEO pack is the best for wordpress

I found All-in-one-seo plugin is good but it leaves tags which tell rivals that I am using it, is there anyone can tell me how to remove it?

<!-- All in One SEO Pack 1.6.12.1 by Michael Torbert of Semper Fi Web Design[409,452] -->

If you want to delete it , you must deactivate or remove plugin.
I think plugin developer deserve a link. isn't it ?