I can think of two first basemen who have worn number 21 in New York Mets history. Lucas Duda is probably the streakiest player I have ever seen. He'd go on these two week tears where it seemed like it was impossible to get him out and then he'd be invisible at the plate for the next month. He's still one of the better first basemen in Mets history.
Carlos Delgado is the other first baseman that comes to mind. He was a key piece of the 2006 Mets when they lost in brutal fashion in Game 7 of the NLCS against the Cardinals. He was a very good first baseman for this team and is a top 15 infielder in Mets history.
Ed Kranepool was the first Met to ever wear number 21 before switching to number 7 in 1965. Todd Frazier wore number 21 during his first Mets tenure. The most recent Met to wear the number is Rich Hill who pitched pretty well after they acquired him at the trade deadline.
Cleon Jones is the best Mets player to wear number 1
When Cleon Jones first came up in 1963 he wore number 6. He struggled in that number so the next time he was up in the bigs he wore numbers 12 and 34 for the 1965 season. After that, he switched to number 21.
In Jones' first season wearing number 21 he took off. He played 139 games and slashed .275/.318/.372 with eight home runs and 57 RBI. He finished fourth in the National League Rookie of the Year balloting and solidified himself as an everyday player for a bad Mets team.
Jones would end up playing 12 of his 13 seasons for the Mets, he played 12 games for the White Sox in 1976 before retiring.
In those 12 seasons, Jones slashed .281/.340/.406 with 93 home runs and 521 RBI. When he retired he was the Mets franchise leader in hits, runs, home runs, RBI, and doubles.
To nobody's surprise, Jones' best season came in 1969. He slashed .340/.422/.482 with 12 home runs and 75 RBI. He tacked on 25 doubles and 16 stolen bases as well. The .340 average is one of the best marks in Mets history and helped lead the Mets to a 100 win season. Jones was an all-star and finished seventh in the National League MVP balloting as well.
Jones is a key reason why the Mets beat the Braves in the NLCS, going 6/14, driving in four runs in the three games. In his other chance in the postseason in 1973, Jones hit .292, over 30 points higher than his regular-season average.
The stats aren't eye popping, but he's one of the most important players the Mets have ever had. He's the one who caught the final out in the 1969 World Series and was one of the best players on that championship winning team. Jones is the best left fielder in franchise history and the best player to ever wear 21 as a Met.