I beleive a good way is checking as many as you can and taking an average. But lets be honest its the SERPs, not the page rank that counts!
I like seobook mozilla toolbar, also shows lots of other information about site besides page rank (i think it is precise) Also use 'SearchStatus' status bar add on on mozilla which shows PR and Alexa ranks.
www.1000dollarsamonth.com works great for me. Don't go for its name. It doesn't ask for captcha and is quick and easy to use. It also has the feature of embeding PR to your site like many others. And best of all, its fast.. as it uses Ajax. Just fetches the rank in split-second.
I use Google Toolbar and it's 100% accurate results shows, please use Mozilla for browser, I use to used other toolbar but they are not showing results correctly Google toolbar is best for me