If you are aiming for a career then pick the most competitive series as high up the ‘ladder’ as you can for your budget in your chosen discipline.
If you are racing for fun then you need to answer a few questions to pick the right series:
What appeals to you? No point spending your hard-earned on a front wheel drive hot hatch if you like historic V12 gt cars.
What can you afford? If you want the experience of driving or racing a particular car then by all means spend your money on that but if you intend staying in something a while and being competitive then race what you can afford to crash, either by having the funds to pay insurance premiums and excess or access to enough money to repair any damage.
Where does it race? Pick the series which visits tracks you want to race at and remember tracks a long way from home increase the budget required.
There are all manner of series out there catering to every taste. What are you racing in 2019 and why did you pick it?