Europe

Bound by the Atlantic Ocean to the North, the Mediterranean and Black Seas to the South and the Urals to the East, Europe is a continent of varying styles of government – some of which are constitutional monarchies and theocracies. It is also home to many contested territories and a few notable microstates.
As responses come in, a link to Anson’s blog post will be in bold.

State
Dependency/Autonomous Region
Title/Sovereign/Head of state
AlbaniaPresident Bajram Begaj
AndorraPrinces Josep‑Lluís Serrano Pentinat
AndorraHead of Government Xavier Espot Zamora
ArmeniaPresident Vahagn Khchaturyan
AustriaFederal President Alexander Van der Bellen
AzerbaijanPresident Ilham Aliyev
BelarusPresident Alexander Lukashenko
BelgiumKing Philippe
BelgiumPrime Minister Alexander De Croo
Bosnia and HerzegovinaChairman Borjana Krišto
BulgariaPresident Rumen Radev
CroatiaPresident Zoran Milanović
CyprusPresident Nicos Anastasiades
Czech RepublicPresident Petr Pavel
DenmarkKing Frederik X
DenmarkQueen Margrethe II (abdicated)
DemarkPrime Minister Mette Frederiksen
Denmark
Faeroe Islands
Prime Minister Bárður á Steig Nielsen
EstoniaPresident Alar Karis
FinlandPresident Alexander Stubb
Finland
Åland Islands
Premier Veronica Thörnroos
FrancePresident Emmanuel Macron
GeorgiaPresident Salome Zourabichvili
GermanyFederal Chancellor Friedrich Merz
GreecePresident Katerina Sakellaropoulou
Greece
Mount Athos
Holy Executive of the Holy Mount Athos – Bartholomew I
Greece
Mount Athos
Elder Abraham, Protos (Spiritual Leader)
HungaryPresident Tamás Sulyok
IcelandPresident President Halla Tómasdóttir
IrelandPresident Michael D. Higgins
ItalyPresident Sergio Mattarella
KosovoPrime Minister Albin Kurti
LatviaPresident Edgars Rinkēvičs
LiechtensteinPrince Alois
LiechtensteinPrime Minister Daniel Risch
LithuaniaPresident Gitanas Nauseda
LuxembourgGrand Duke Henri
LuxembourgPrime Minister Luc Frieden
MaltaPresident Myriam Spiteri Debono
MoldovaPresident Maia Sandu
MonacoSovereign Prince Albert II
MonacoMinister of State Pierre Dartout
MontenegroPresident Milo Đukanović
NetherlandsKing Willem-Alexander
NetherlandsPrime Minister Dick Schoof
North MacedoniaPresident Stevo Pendarovski
NorwayKing Harald V
NorwayPrime Minister Jonas Gahr Støre
PolandPresident Andrzej Duda
PortugalPresident Marcelo Rebelo de Sousa
RomaniaPresident Nicușor Dan
RussiaPresident Vladimir Putin
San MarinoCaptains Regent
SerbiaPresident Aleksandar Vučić
SlovakiaPresident Peter Pellegrini
SloveniaPresident Nataša Pirc Musar
SpainKing Felipe VI
SpainPrime Minister Pedro Sánchez
SwedenKing Carl XVI Gustaf
SwedenPrime Minister Ulf Kristersson
SwitzerlandFederal President Viola Amherd
TurkeyPresident Recep Tayyip Erdoğan
UkrainePresident Volodymyr Zelenskyy
United KingdomKing Charles III
United KingdomPrime Minister Keir Starmer
Vatican CityPope Leo XIV
Vatican CitySec of State Pietro Parolin

Other Territories & Microstates

State/Territory (Country)Leader (title)
Abkhazia (Region of Georgia)Badra Gunba  (President)
Freetown Christiania (Denmark)Christiania Fonde
Gagauzia (Moldova) Evghenia Guțul (Bashkan of Gagauzia)
Olivenza (Spain)Manuel José González Andrade (Mayor)
Principality of SealandMichael Bates (Prince of Sealand)
Republic of Užupis (Lithuania)Romas Lileikis (President, symbolic)
Republika Srpska (Bosnia and Herzegovina)Milorad Dodik (President)
Savage Islands (Portugal)Comandante da Guarnição nas Ilhas Selvagens
South Ossetia (Region of Georgia)Alan Gagloev (President)
Transnistria (Moldova)Vadim Krasnoselsky (President)