Question: Vykintas was a rival to which first Grand Duke of Lithuania?

Context paragraphs:
Daumantas of Lithuania: Daumantas or Dovmont was the Grand Duke of Lithuania in 1282–1285.  Daumantas is mentioned in chronicles only once and, in absence of any other evidence, is presumed to be a short-ruled Grand Duke who inherited the title after Traidenis' death in 1281 or 1282.  It is assumed that Daumantas was succeeded by Grand Duke Butegeidis.  Relationships between Traidenis, Daumantas, and Butegeidis are unknown.

Union of Horodło: The Union of Horodło or Pact of Horodło was a set of three acts signed in the town of Horodło on 2 October 1413.  The first act was signed by Władysław II Jagiełło, King of Poland, and Vytautas, Grand Duke of Lithuania.  The second and third acts were composed by the Polish nobility (szlachta) and Lithuanian boyars respectively.  The union amended the earlier Polish–Lithuanian unions of Krewo and Vilnius–Radom.  Politically, Lithuania received more autonomy as, after the death of Vytautas, the Lithuanian nobles could choose another Grand Duke instead of passing the title to Władysław II Jagiełło or his heir.  However, culturally, Lithuania and Poland grew closer.  Lithuania adopted Polish institutions of castellans and voivodes. Catholic Lithuanian nobles and church officials were granted equal rights with the Polish nobles and clergy.  Forty-seven selected Lithuanian nobles were adopted by Polish families and granted Polish coats of arm.  Thus the union signified the beginnings of Polonization of Lithuanian culture and the rise of the Lithuanian nobility.  It was one of the major steps towards the modernization and Europeanization of Lithuania.

Vykintas: Vykintas (died ca. 1253) was Duke of Samogitia and a rival to the future King of Lithuania, Mindaugas.  In 1236 he probably led the Samogitian forces in the Battle of Saule against the Livonian Order.  The Order suffered a great defeat and was near the brink of collapse, forcing it to become a branch of the Teutonic Knights.

Andrei of Polotsk: Andrei of Polotsk (Belarusian: Андрэй Альгердавіч , Lithuanian: "Andrius Algirdaitis" Polish: "Andrzej Olgierdowic" , ca. 1325 – 12 August 1399, in the Battle of the Vorskla River) was the eldest son of Algirdas, Grand Duke of Lithuania, and his first wife Maria of Vitebsk.  He was Duke of Pskov (through his deputy Yuri, 1342–1348) and Polotsk (1342–1387).  As the eldest son of the Grand Duke, Andrei claimed his right to the throne after his father's death in 1377.  Algirdas left Jogaila, his eldest son with his second wife Uliana of Tver, as the rightful heir.  Andrei's rivalry with Jogaila, Grand Duke of Lithuania and later King of Poland, eventually led to his demise.

Lithuanian Civil War (1389–92): The Lithuanian Civil War of 1389–92 was the second civil conflict between Jogaila, King of Poland and Grand Duke of Lithuania, and his cousin Vytautas.  At issue was control of the Grand Duchy of Lithuania, then the largest state in Europe.  Jogaila had been crowned King of Poland in 1386; he installed his brother Skirgaila as ruler of Lithuania.  Skirgaila proved unpopular and Vytautas attempted to depose him.  When his first attempt to take the capital city of Vilnius failed, Vytautas forged an alliance with the Teutonic Knights, their common enemy – just as both cousins had done during the Lithuanian Civil War between 1381 and 1384.  Vytautas and the Knights unsuccessfully besieged Vilnius in 1390.  Over the next two years it became clear that neither side could achieve a quick victory, and Jogaila proposed a compromise: Vytautas would become Grand Duke and Jogaila would remain Superior Duke.  This proposal was formalized in the Ostrów Agreement of 1392, and Vytautas turned against the Knights.  He went on to reign as Grand Duke of Lithuania for 38 years, and the cousins remained at peace.

Isabella de' Medici: Isabella Romola de' Medici (31 August 1542 – 16 July 1576) was the daughter of Cosimo I de' Medici, first Grand Duke of Tuscany, and Eleonora di Toledo.  She was raised and educated in a humanist manner with her siblings, including Francesco de' Medici who succeeded their father as the Grand Duke of Tuscany.  To secure a relationship with the powerful Roman Orsinis, Isabella's father arranged her marriage to Paolo Giordano I Orsini when she was 16.  She remained in her father's household after her marriage, giving her an unusual degree of independence for a woman of her period.  Following the death of her father, Isabella was probably murdered, with the complicity of her husband and brother, and in retribution for her relationship with Paolo Giordano's cousin Troilo Orsini.

Family of Gediminas: The family of Gediminas is a group of family members of Gediminas, Grand Duke of Lithuania (ca. 1275–1341), who interacted in the 14th century.  The family included the siblings, children, and grandchildren of the Grand Duke and played the pivotal role in the history of Lithuania for the period as the Lithuanian nobility had not yet acquired its influence.  Gediminas was also the forefather of the Gediminid dynasty, which ruled the Grand Duchy of Lithuania from 1310s or 1280s to 1572.

Ostrów Agreement: The Ostrów or Astrava Agreement (Lithuanian: "Astravos sutartis" , Belarusian: Востраўскае пагадненне , Polish: "Ugoda w Ostrowie" ) was a treaty between Jogaila (Władysław II Jagiełło), King of Poland and Grand Duke of Lithuania, and his cousin Vytautas the Great, signed on 4 August 1392.  The treaty ended the destructive Lithuanian Civil War, launched in 1389 by Vytautas who hoped to gain political power, and concluded the power struggle between the two cousins that erupted in 1380 after Jogaila secretly signed the Treaty of Dovydiškės with the Teutonic Knights.  The Ostrów Agreement did not stop attacks from the Teutonic Knights and the territorial dispute over Samogitia continued up to 1422.  According to the treaty, Vytautas became the ruler of Lithuania (styled Grand Duke), but he also acknowledged Jogaila's (styled Supreme Duke) rights to Lithuania.  The details of the Polish–Lithuanian relationship were clarified in several later treaties, including the Union of Vilnius and Radom in 1401 and Union of Horodło in 1413.

Mindaugas: Mindaugas (German: "Myndowen" , Latin: "Mindowe" , Old East Slavic: Мендог — "Mendog" , Belarusian: Міндоўг or Mindoūh , c. 1203 – autumn 1263) was the first known Grand Duke of Lithuania and the only King of Lithuania.  Little is known of his origins, early life, or rise to power; he is mentioned in a 1219 treaty as an elder duke, and in 1236 as the leader of all the Lithuanians.  The contemporary and modern sources discussing his ascent mention strategic marriages along with banishment or murder of his rivals.  He extended his domain into regions southeast of Lithuania proper during the 1230s and 1240s.  In 1250 or 1251, during the course of internal power struggles, he was baptised as a Roman Catholic; this action enabled him to establish an alliance with the Livonian Order, a long-standing antagonist of the Lithuanians.  During the summer of 1253 he was crowned King of Lithuania, ruling between 300,000 and 400,000 subjects.

Lithuanian Civil War (1381–84): The Lithuanian Civil War of 1381–1384 was the first struggle for power between the cousins Jogaila, Grand Duke of Lithuania and later King of Poland, and Vytautas the Great.  It began after Jogaila signed the Treaty of Dovydiškės with the Teutonic Knights which was aimed against his uncle Kęstutis, father of Vytautas.  Kęstutis briefly seized power in the Grand Duchy, but was betrayed by adherents of Jogaila primarily from Vilnius.  During negotiations for a truce Kęstutis and Vytautas were arrested and transported to the Kreva Castle.  Kęstutis died there a week later but Vytautas managed to escape and then sought an alliance with the Teutonic Knights.  Subsequently their joint forces raided Lithuanian lands.  Eventually the cousins were reconciled as Jogaila needed internal stability in anticipation of negotiations with the Grand Duchy of Moscow and the Kingdom of Poland regarding the possible Christianization of Lithuania.  The war did not settle the power struggle; it continued during the next Lithuanian Civil War (1389–1392) which was resolved by the signing of the Ostrów Agreement.  After more than ten years of struggle, Vytautas finally became the Grand Duke of Lithuania and ruled the country for thirty-eight years.

Supporting paragraphs: Vykintas Mindaugas
Answer: Mindaugas