BERLIN, Sept 26 (Reuters) - Germany"s CDU/CSU conservatives and their Social Democrat rivals were tied in Sunday"s national election, an exit poll showed, leaving open which of them will lead the next government as Angela Merkel prepares to stand down after 16 years in power. The CDU/CSU bloc won 25% of the vote, their weakest result in a post-war federal election and on a par with the centre-left Social Democrat (SPD), the infratest poll for broadcaster ARD showed. Attention will now shift to informal discussions - likely with the Greens and liberal Free Democrats (FDP) - followed by more formal coalition negotiations, which could take months, leaving Merkel in charge in a caretaker role.