Perhaps unsurprisingly
I use a few car body products for big gaps. A spoonful from a tin at work lasts forever at our scale. Being 2 part chemically cured they never sink either.
My industry is fast moving into UV cured products for speed & energy saving so I tried out my Colad UV putty on a few bits recently. Slap it in thick, a few seconds with a UV lamp and it's ready to sand. Probably a bit hard for our work but could be great for filling large gaps very quickly before a final skim with something easier to sand. The curing lamps are a bit spendy though! I have a couple of small Unilite ones that do the job but my "big boy" costs £1500
Back in the real world of modelling products I like Mr White Putty in a tube for thicker stuff then Mr Dissolved Putty for thin or shallow areas. Being a thick liquid it seeps into small cracks and levels out minor imperfections well. They both tend to bite into the plastic better than waterbased acrylic fillers, so feather out more easily.