Professionals?
I think we're all pretty amateur mate!
(does mollusc qualify as he actually makes a few quid from tiny plastic things...)
Squadron stuff's the best in a tube, there's some relatively new style fillers that I find much easier to use though, 'Mr Surfacer' & a Vallejo version that come in standard paint containers, they're more like thick paint and much easier to sand and apply.
Another handy (and free!) filler is bits of old sprue melted down in an old pot of liquid glue, it's great for filling tiny gaps like joints on figures limbs but has to be used a little cautiously as too much of it can melt the surrounding plastic.