Well, barely anyone wants Friedrich Merz as next chancellor, but his CDU (+ bavarian partner CSU) are really bound to win the election with 30+ % unless Merz finds ways to make himself yet more unpopular even with CDU voters. He will be their candidate after his biggest rivals, Prime Minister Wüst of NRW and Markus Söder of Bavaria, conceded the candidacy to him a couple of months ago.

Friedrich Merz is going to be Germany's next chancellor unless videos surface of him killing kittens for satanic rituals in preparation of holding an orgy in which he is costumed as Vladimir Putin whipping away at someone costumed as Donald Trump. That would gain him lots and lots of votes from AfD people, but probably still lose him votes overall.

A more popular alternative from another party would be Boris Pistorius, currently defence secretary, but SPD are bound to re-run with Scholz on this short notice and Pistorius said he isn't available. Even if Pistorius was available, personal popularity only gets you so far if people don't want your party to lead the next government.

The first preference for a coalition partner of CDU/CSU would absolutely be the FDP, but there's some work to do there, some percentages to gain. It is however somewhat within reach. CDU+CSU+FDP+Greens would easily have enough seats, but someone would have to drug the heads of FDP and Greens to give close cooperation another go so soon, although these three parties would be the most similar on Ukraine policy.

Favourite for next government is easily a grand coalition of CDU/CSU + SPD. Given that a bunch of parties don't get seats because of Germany's 5% hurdle, those two parties should easily get enough seats for a majority even if they don't reach 50% together. CDU/CSU + Greens are an option that Merz has started being open to it, but pretty sure SPD would be preferred and Söder (leader of CSU) has done a lot of work stylising the Greens as Enemy No. 1 this summer.

If unexpectectly CDU/CSU and SPD don't get enough seats for a majority, then first choice for a third partner would be the FDP (if they scrape over the 5% hurdle), followed by Greens.