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Greece is Western Europe in this context (and I doubt Turkey would accept a Greek SG). As mentioned by another poster earlier Kolinda Grabar-Kitarović (former Croatian President) and Dalia Grybauskaitė (former Lithuanian President) are considered likely options. But Poland definitely has a good claim to the post given its size and geo-strategic importance.
Kolinda Grabar-Kitarović has a formidable CV and would be a great contender on paper: lived in the US for several years, knows 7 languages on varying degrees of fluency (Croatian, English, Spanish, Portuguese, German, Spanish, Italian), served as Minister for European Affairs & Foreign Affairs, represented Croatia as ambassador in the US, was involved in NATO as Assistant SecGen, became President and has been Croatian representative in the IOC since her election loss. Stoltenberg lost an election as well before becoming NATO GenSec, so that wouldn't be an issue probably, it's just the circumstances surrounding her loss in 2020.
KGK was quite popular for most of her term (many might still remember her for being a big fan of the Croatian team in the 2018 soccer world cup), although some of her comments on typical Balkan affairs (Serbia, Bosnia...) might have harmed her reputation as a diplomat. Being confronted with an unpopular government and a challenge from the far-right, KGK tried to appeal to right-wing voters, losing much of her credibility. She hold a completely chaotic speech shortly ahead of the election in which she promised jobs with 8000 euro salaries, talked gibberish ("I am not my program that is you the Croatian people in Croatia and in homeland"), praised Luka Modrić as "her son Lukica" and lost any credibility she had before (nickname Alkoholinda). To her party HDZ she has been a sorta persona non grata ever since.
Following her loss, she received some attention for showing the middle finger to the right-wing politician who came close to knocking her off the runoff after he stated he would want to ban abortion completely even in cases of rape.