Hi DanielThePrawn, welcome aboard.
I can't speak for the best Airfix tank for a beginner, but unless I am misunderstaning you, if you are a new modeller I'm not so sure that an Airfix tank is the best place to start modelling?
I would say that Airfix tanks can be something of a challenge as they are pretty much old mouldings which tend to be worn and very much of the era they were born in to anyway with some inaccuracies and at times questionable fit. I'd hate for a new modeller to give up in frustration and never bother again.
I recognise you might want to build a tank, on the other hand, if you are a new modeller, I might point you to some of the new-tool Airfix releases (albeit aircraft) so that you can get your hand in, putting what you've learned into practice with older and less favourable kits. You might not find them as frustrating as a kit that might need more work than you envisage.
If you are more experienced than I am reading into your post I apologise; someone no doubt will have a better idea of the better of the tanks.
I'd be fine with getting a kit for a plane. If the tank kits are of lower quality then i'll most likely get a plane kit. Thanks for your help