The first determining factor is if you have 5 or 6 wheel studs.
For 5 stud rims, singles choices are getting more and more limited as time passes, as these are what came on older model trucks and companies that used to provide them no longer do.
For 6 stud rims, your choices these days are normally 17" and 19.5".
17" rims normally have a better selection of tyre choices and the ride will likely be slightly better on these tyres, as the sidewalls are normally softer. Weight carrying capacity would be the biggest issues with these tyres.
The 19.5" tyres are normally light truck tyres, so they are much stiffer in the sidewall. These tryes are normally more robust and can carry a greater load, but the ride is firmer and they don't air down as well.
Basically, it really comes down to what you will be doing with your truck.
Be mindful that if someone says "I love these tyres... they are the best" that may well be true on their own truck, but they may be totally inappropriate on your truck.
The other significant factor is where you are located, as rim and tyre choices are not the same globally.
Arguably, Australia probably has the greatest selection of singles rims, but we don't have a lot of options when it comes to tyres.
