Jack and Billy are for me way above the others and no doubt will pick up most of the votes here. So I'm voting tactically for Millie who has put in some great performances this month against good players.
Any other month and Jack would be the outstanding favourite, with his first ATP250 title. Julian had a really good month (if "only" consistently repeating previous highs). Sonay getting through Wimbledon qualis was great. Millie made some fantastic progress.
But Billy really did have a career-changing month and it was the whole month, not just a good week or two. So Billy it is!
17 votes in so far; remember we close at 11 pm tonight. And also looking at the voting, not sure there is much space for tactical voting here, so think before being tempted!