If you want the longest track record and the most established user community, Aviationexam or BGS are the conventional choices. They have been in the market longer and many instructors know them.
If you want per-question LO mapping, built-in spaced repetition, offline PWA access, and pricing based on revision duration rather than a monthly rolling cost, SkyStudy is worth evaluating, keeping in mind it is newer and does not have independent pass-rate data yet.
If you are a flight school looking for cohort analytics, SkyStudy's beta dashboard is the only one of these four with explicit instructor-facing tooling, though it is still in testing. See SkyStudy for flight schools for the honest description of what is available now.
All four platforms are study aids. None of them replaces a good ground school, competent instruction, or studying official EASA source material.