Not exactly your question but it's relevant to it and a bit long for a comment. Schengen visas never allow work in the whole area, nor do they forbid it as such. But if you are from outside the EU and want to take up employment in a Schengen country, you need some form of authorisation to work from that particular country, on top of the authorisation to enter and stay in the Schengen area granted by your visa.
Typically, such an authorisation comes in the form of a national visa or residence permit but if you don't need one (e.g. because you will stay less than 90 days under a Schengen visa or a residence permit from another Schengen country), you still need a separate authorisation to work. See https://travel.stackexchange.com/questions/44265/internship-in-germany-on-normal-schengen-visa for some details on how it works in one such situation.
From your comments, it seems that what you have is not a Schengen visa but a Polish national visa. That visa might allow you to work in Poland (some student visas do, e.g. up to a yearly maximum number of hours) and to travel in the Schengen area but not to work elsewhere than Poland.
Consequently, if you want to find a part-time job in another EU country as a student, it will probably be necessary to switch universities as well to qualify for the right type of visa. Which country would be best for that, I don't know, but since it would require moving there semi-permanently, you need to consider other factors than work as well (housing, tuition…)