My choices:
G: 1955/56 Bob Cousy
G: 1958/59 Larry Costello
F: 1955/56 Paul Arizin
F: 1959/60 Bob Pettit
C: 1959/60 Bill RussellBob Cousy - probably the best guard season available and arguably Bob's overall peak. He was still quite efficient (around league average at high volume) and his passing ability was excellent. He got massively worse after 1958 in my opinion, but this season is an excellent choice. People know more about Cousy than most players from that era, so I won't talk too much about him here.
Larry Costello - I have to admit that this era is weak for guard position, mainly because how coaches used guards on offense in the 1950s. Still, it's not without talent and Costello had fantastic season in 1959. He posted nice statline of 16/5/5 on +4.6 rTS% and he played even better in playoffs with excellent 18/6/6 +8.1 rTS%. If I had to choose one guard season better than 1956 Cousy, it would be this Costello performance.
I also had in mind Richie Guerin, Bill Sharman, Carl Braun, Gene Shue and Tom Gola. There will be many tough decisions in next rounds.
Paul Arizin - this is fairly easy choice, Arizin had one of the finest offensive seasons of this era including one of the best postseason runs ever. I'd call Arizin the best scorer of this era with Pettit being the only other candidate: 24 ppg on +6.9 rTS% in RS and
29 ppg on +7.3 rTS% in playoffs finished by winning the title. 1956 Warriors had strong team, so it's not like he carried them but his best teammate (Neil Johnston) did underperform, so Arizin had to up his scoring numbers.
Paul Arizin was known for his signature high release midrange jumpshot, but he also had deep range and could slash to the basket well. He was known for his jumping ability and overall athleticism, which helped him being very effective offensive player despite small statue (only 6'4 without the shoes).
Bob Pettit - another easy choice, Pettit is one of the best players ever at his position. Excellent rebounder, strong scorer and good shooter. He wasn't ball-dominant and he thrived with other stars like Hagan or Lovellette.
Bill Russell - the easiest choice, Russell already reached his prime in 1959 and he had probably his best scoring season in 1960 while already being GOAT defender.