Well almost all blog scripts have url rewrite/seo friendly links, ping/trackback . But if you ask me, I would recommend wordpress as its the one I know most, and I believe its "Permalink" feature is marvelous where you can easily change the URL format.
Wordpress seems to be more SE friendly from others however if you are using other software you can make them search engine friendly too.
blogger is great as it requires no mods etc. It's straight out the box + you can earn adsense on it. All free. Wordpress is more flexible blogging, but requires mods for SEO. + you can use google ads on the freely hosted wordpress blog. doh!
Well it really depends on your goals, also depends if you want a hosted script or if you want something you can use on any site. Either way I like Wordpress the best, though I do have a few blogger blogs that feed to other sites that gets mass amounts of traffic from Google and Yahoo. Mind sharing or pointing us in the right direction to these tweaks.
I have an e-book about SEO for Wordpress blogs. If you want it, PM me and I'll e-mail it to you (it's not an e-book I wrote, but one I got for free).
You say that blogger doesn't need any SEO mods. I have a question regarding that statement: 1 - You mean, new BETA BLOGGER right ? Tahnks a lot !!!
Wordpress is the best one. It has more user friendly features when compared to blogspot. I have no idea about livejournal and myspace.
Blogger, being owned by google, gets crawled a lot, so if you are looking for a free hosted blog I would use blogger. (I have many of my own blogger blogs for backlinks.) However, if you are hosting yourself, I wholeheartedly recommend Wordpress. I think it is easily the most user friendly, with many great SEO plugins.