Personally I permaban Vex every game because she's just the most infuriating matchup imo, but Syndra is by far the absolute worst matchup for Jayce, with Irelia being a close second.
Fizz because he can easily burst, dodge, and close the gap fast. Yasuo because he can easily dodge you abilities or block and dash to close the gap. Zed because he can just easily pop you. This is just my opinion though.