I'd say its debatable but Ferrari, Williams or Toyota (IMO in that order) have the most powerful engines, followed by mercedes. Then probably BAR and the cosworths, Renault's engine isn't actually that powerful.
Reliability is a different issue though. My guess is McLaren have risked putting high risk parts on thier MB engine to get more power from it in the hope to try and beat ferrari this season. Hopefully it'll be more of a level playing field next year and the top teams will all have more equal reliability as it was before this year.
and bernie on the 3 car issue:
http://www.itv-f1.com/News_Article.aspx?PO_ID=31140