For low power, C6-5. Every 18mm rocket I fly gets a C6-5. Buy it in bulk and fly it in bulk.
For 24mm, my 2 go to motors are the F39 and F24. Good bang for your buck. My Estes Hojo screams with a F39.
The G76G or G64W is my go to motor for mid power rockets. Good bang for your buck, and it fits most of my 29mm rockets. I could send my heavier ones to 1k or my sport birds shy of 4k. I don't even want to know how many of each I've burned.
For my lower side high power rockets, I love the H128W or H165R. Sometimes the H238T as well. Good bang for your buck and works for most of my rockets. Definitely my go-to motors
For medium size high power, you can't go wrong with the 38/360 loads, specifically the I245G. I absolutely love it. If I hadn't veered off into more EX stuff, I would only fly I245s. My first 38mm motor was a I245. So was my second, third, and fourth...
For the longer 38mm motors, I love the J350W. Works great in heaver 38 MMT birds and is a great motor to adapt a 54 MMT to. My go-to motor for a gentle flight on some of the shinier rockets. The J570W is also great for pushing some of my Wildman Jr kits. Definitely my go-to motor for altitude.
For the 54mm range, you can't beat a K550W. However, I do tend to launch a larger variety of propellants in 54mm range. If I had to pick one, definitely the K550W.
For larger, I just change it up each time. I got a L850W on order right now and plan to try out the L1256SW and L1940X for the first time after the black Friday sales. L850W and L900DM are honorable mentions, and would be my go-to if I was limited to only one motor.