While the answer will vary based on perspective trader risk capital, risk tolerance, personality etc. I do think that there are a few markets that might be a better start for first time futures day trader.

I personally would say, leave the mini SP alone. yes it has the biggest volume but there is quite a bit of size on the bid/ask that may make this frustrating for new traders.

My favorite markets to share with first time traders are:

1. mini Dow
2. ten year notes/ 30 year bonds
3. mini crude/ mini gold

The mini Dow moves similar to mini sp but the value per point/tick is smaller, still has good volume but not as hard to get filled on the limits as the mini SP

The 10 year notes are usually less volatile than the other markets mentioned. Completely different type of trading personality and offer a good diversification option for new traders and experienced traders. The 30 year bonds will move a bit more than the 10 years and are a bit more volatile but very similar. If you are moving from beginner to intermediate, trading the spread between the 10 years and the 30 years is an option to explore!  I included a chart of both for reference below.

The mini gold and mini crude are another good option because it is the smaller contract size of two markets that can really move, offer volatility and RISK but the availability of the mini sized contracts make these two a better option for beginners until one has experience / risk capital and appetite for the standard contract sizes of gold and crude oil.

Ten Years and 30 years 10 minutes chart:

Custom TYA – 10 YR US Treasury Notes (Globex), Equalized Active 10 Min Continuation