I've three to choose from:
1st solo - C150 at Leavsden while on an RAF Flying Scholarship at Luton in 1977. I had been having issues with landing (frightened of hitting the ground hard, which made it far, far worse. Solved by a change of instructor, and I was fine thereafter).
1st military solo - Chipmunk at Roborough on FAA Flying Grading. A lovely bright but not breezy November day, and on the previous circuit I was shown where to dump it if I had an engine failure on crosswind/early downwind. I remember feeling very good during and after. Best fun with one's trousers on.
1st rotary solo - Predannack in a Gazelle. Wonderful aircraft, and only wish I could run one now
Of the three it's the Chipmunk trip I remember the best and with the fondest memories.