Fibre glass side panels

What is the best paint to use on fibre glass side panels, that don't crack in a few years.

Any good quality automotive basecoat/clearcoat will work fine on properly prepared and epoxy-primed fiberglass. Most of these are acrylic/urethane enamels.

The bulletproof solution would be to use a 2K filler/primer with plastisiser.
If spraying over crazed fibreglass I'd recommend grinding back into the cracks and filling (I used superglue last time) to prevent future issues - a smooth outer surface won't fix the delamination which has occurred in the substrate between the matting and the gelcoat - the cracks are just the surface evidence of underlying issues, and could easily re-surface if not properly addressed IMHO.

With all the fibergalss I've painted, the paint itself has never been an issue. Standard 2K primer and any good topcoat system is fine. It's those damn cracks, especially the right cover around the two top mounting holes, that have to be dealt with first.