Although your flying career has only one 'first solo' which you never forget there are several other significant equivalent 'first solo' flights along the way.
My first 'first solo' was in a PA28 Cherokee on 30Nov1973 at Bankstown near Sydney. Next was first solo in a Winjeel after I joined the RAAF, then first solo in a Macchi MB326 jet trainer followed by first solo during my helicopter course in a UH1H Iroquois. All these took place in a single place aircraft, though on the helo flight we all had a course mate come along for the ride. Brave man.
All the other types I've flown have been multi-crew aircraft but your first trip in command in them is always a memorable event and a lot more involved than a lap around the circuit. For example my first command trip in a Herc was a multi sector day with eight hours of flying, some at night and several instrument approaches.
Later in my airforce career my first command trip in a B707 was a multi-sector trip over a week, up through the tropics, to Butterworth in Malaysia. The first day of which was like a sim ride from hell with all manner of things going wrong and resulting in a one engine INOP landing in Butterworth.
The next first solo wasn't until years later after I'd left the RAAF and joined my present airline and, on my first trip in command, was faced with an approach right to m . m . minima in a driving tropical rainstorm, in Cairns, followed on the next sector by another approach right to minima at night in a snowstorm in Narita.
These things you never forget but nothing replaces the feeling of your very first solo flight.
Regards,
BH.