The nine timestable was one of the easier ones for me to learn as a child because I recognized the pattern in the numbers:

• 09
• 18
• 27
• 36
• 45
• 54
• 63
• 72
• 81
• 90

First row of digits go 0123456789 while the second row goes 9876543210. Idk, I thought it was cool that they did that.

The timestables that I had the most difficulty with were 7 and 8.