The 41-year-old established himself as one of the game's finest shot-stoppers during his illustrious spell at Real Madrid, making 725 appearances for the club in all competitions.
Despite his relatively small 5ft 11in frame, Casillas recorded 264 clean sheets in 16 years as a senior Real Madrid player, winning a total of 19 major trophies in that span.
The ex-Spain international - who won the World Cup and Euros with his nation - collected three Champions League crowns and five La Liga titles before ending his career in 2020 after a brief spell at Porto.
Casillas - who suffered a heart attack in 2019 - recently named his list of top-five goalkeepers to 433, but there was no spot for Liverpool number one Alisson.
The Brazil international has starred for Liverpool since arriving from Roma in 2018, winning the Champions League and Premier League during his time at Anfield.
Alisson has endured a challenging start to the new season, though, keeping just two clean sheets in seven appearances and conceding 10 goals in all competitions.
The Liverpool man has been involved in a lengthy battle with Manchester City's Ederson for the Brazil jersey, and Casillas has listed Alisson's compatriot as the world's second-best goalkeeper.
The 29-year-old won the Premier League's Golden Glove award for the third consecutive season last year - keeping 20 clean sheets alongside Ederson and winning his fourth Premier League title.
At the top of Casillas's list was the man is current Real Madrid number one Thibaut Courtois, who ousted the Spaniard's successor Keylor Navas from the Real XI upon his arrival in 2018.
The Belgium international was named in both the Champions League and La Liga Team of the Season lineups last year and made an astounding nine saves in the European Cup final against Liverpool.
Atletico Madrid's Jan Oblak occupies third place on Casillas's list, with the Slovenia international helping Diego Simeone's side to La Liga glory in 2021 and winning five Ricardo Zamora Trophies for the lowest goals-to-games ratio in La Liga.
With Courtois, Ederson and Oblak in the top three, Barcelona number one Marc-Andre ter Stegen - who has won four La Liga titles and the Champions League with La Blaugrana - finds himself fourth in Casillas's list.
The fifth and final position belongs to Bayern Munich and Germany veteran Manuel Neuer, who is still going strong at 36 and won his 10th consecutive Bundesliga title with the Bavarians last term.