I have tried all
four of Plesk (v2.5 and v5.0.3), cPanel (from v4 to v6.2), Ensim (old one though) and H-Sphere (old too I think).
Personally, my favourite would be a hit between cPanel and H-Sphere. I like Plesk, although it does lack a lot of features - and for the things my clients need and want, I absolutely have to give cPanel.
Personally, I cannot wait until v6 of PlesK comes out, as then there are supposed to be a lot more features and it's supposed to be a lot better. The only things I want, is for my clients to be able to create their own sub-domains without hassle and without me giving them a reseller account, as well as for them to be able to do Cron Jobs without hassle and without me giving them SSH which I don't want to do.
Plesk is nice. I'd actually prefer to use that. Although currently because of the clients, it seems that cPanel is the winner.
Although what I do plan to do to cPanel soon is make a custom skin for my clients that will look really nice and easy for them to use. I'm hoping it'll get under way soon, but only time will tell.