Former Tottenham Hotspur star Gareth Bale has been hailed as the most the talented player Allen has played with.
Allen has played with Bale for the Welsh national side and has backed his countryman to improve even further after a successful first season in Madrid, where he helped Los Blancos win their tenth European Cup, scoring a crucial goal in extra time.
In an interview with Liverpool's official programme, Allen said: "He's still going to get better and better which is frightening really. He's the world's most expensive player and has done a fantastic job since he moved to Real Madrid. People focus a lot on players' first seasons with teams and for him to go abroad to a massive club like that and lift the UEFA Champions League in his first season just shows you how good he is and how good he's going to be."
Allen has been lucky enough to play with some top class players so far during his career - and the fact that Bale stands out ahead of Liverpool captain Steven Gerrard and former teammate Luis Suarez demonstrates the quality that Bale possesses.
He is likely to come up against his fellow Welshman in the Champions League when Liverpool take on Real Madrid in Group B of the Champions League. The two sides meet at Anfield on October 22, with the return game at the Bernabeu on November 4.
Both sides got off to a winning start in the European campaigns, with Real thumping Basel 5-1, and Liverpool overcoming Ludogorets 2-1.