Should I only pick universities on the global table? Or wouldn’t it make any difference if I chose a nationally ranked one?
First you need to take a step back and consider what criteria each ranking is ranking universities on. Is it a ranking of research output? That’s only significant for undergraduates if you want to attend a PhD program afterwards. Is it a ranking of teaching quality? That is relevant for undergraduates, but only insofar as the college experience is concerned; it wouldn’t tell you much about your “employability” afterwards. If the ranking is based on an opinion survey, consider who was surveyed. Do you case about the opinion of that particular group of people? If it’s a ranking based on alumni outcomes (e.g. income 10 years after graduation), keep in mind that incomes are highly correlated with career choice. Universities with large pharmacy programs do consistently well on income rankings, as do “trade schools”. Universities that train a large portion of scientists (PhDs) or human service occupations do worse than expected because those occupations tend to pay less than similar “corporate” occupations.
If you expect to enter the workforce after finishing your university-level education, then the opinion of your potential employers is what matters. Ask around and find out if employers in your country care about where you study.
If you plan on going overseas for work going to school which is ranked in the global table would surely help you. But if you have no intention on leaving your country nationally ranked ones would be enough. But may I ask why are you aiming for a ranked school?
If you plan on going to grad school in the US, national rankings and especially wrt to PHD production are important. For career purpose, career placement, alumni network, etc., matter more than ranking.
For an undergrad, I prefer the national rankings because the international rankings essentially look at info that matters to grad students, esp. research output, and do not rank LACs (and so ignore top-notch, uniquely American institutions). In addition, rankings should be used cautiously, in groups of 20 or do. Within a grouping, there’s virtually no quality difference - there differences are often subjective (core vs. open curriculum, consortium or not, city vs. college town vs. rural, big spectator sports or big intramural participation, outdoorsy, intellectual, preprofessional…)
Try to acquire one or two of the following: Princeton Review’s best colleges; Colleges that change lives; Fiske Guide; Insider’s guide to the colleges; Colleges that pay you back. You’ll get a much better picture of the higher education landscape in the US.