The pre-clinical years (usually the first two years, but many schools are condensing this phase into 1 or 1.5 years) are very meritocratic in that your standing depends on mostly multiple choice tests. And then you take the first medical licensing exam called Step 1 which largely determines your competitiveness for residency programs. The clinical years where you are actually working in the hospital in various fields is less objective as you are judged based on the subjective feedback evaluations of the residents and attending physicians.

However, you are mostly rated on how much you know, how confident you are and your social skills. So if you did really well in the pre clinical years and are not socially inept then you’ll probably get good evaluations.

But it’s a marathon. It’s an unbelievable amount of information you have to memorize and in the beginning it’s hard to stay motivated when you can’t see it’s direct relevance.

So overall I’d say it’s fairly meritocratic. If you can memorize things well and are social then you’ll do well barring any exceptional circumstances.