The Former Yugoslav Republic of Macedonia could meet the practical, legal and technical criteria for joining the bloc in the next six years, the study found.
But the other official candidates including Albania, Macedonia, Montenegro, Serbia and Turkey, and EU hopefuls Bosnia-Herzegovina and Kosovo are well off the pace.
The Forecasting Candidate Status study, by Professor Tina Freyburg of the University of St Gallen, considered how likely it is that five of those candidate countries would join the EU before 2050, based on previous accession benchmarks used during the 2004 big-bang enlargement.
Accession is only possible when candidate countries meet a comprehensive list of criteria and when they have implemented EU law into their own national legislation.
This includes everything from judicial reform to transport and energy policy.
Prof Freyburg found Turkey and Serbia – currently touted as the frontrunners together with the tiny coastal republic of Montenegro – would only manage to fully comply with EU law in the mid-2030s.
Turkey’s long-running membership bid is the most controversial, not least because of President Recep Tayyip Erdogan’s crackdown on opposition forces following a failed coup last year.
Member states have urged Brussels to suspend or even scrap Turkey’s bid which was formally launched in 1987.
Albania and Bosnia-Herzegovina might not be in a position to meet the criteria on EU accession before 2050, which the study uses as evidence to show that political ambitions in those countries might outweigh the reality of the situation.
Montenegro’s prospects of successful accession are often hailed in Brussels as the most promising.
Factors including a border dispute with Kosovo continue but its barriers to EU membership are less significant than other candidates.
Prof Freyburg said: “Montenegro might reasonably be seen as being among the first potential next member states.”
But she warned the political willingness of the Montenegrin government and vetoes by other member states are outside the control of the study.
The study also highlighted that enlargement is most often driven by candidate country action and progress, rather than an “expansionist” Brussels policy.
It added EU concerns about increased membership jeopardising a “deepening of the European institutions” are non-technical barriers that prospective members must contend with.
The latest addition to the EU was Croatia, which formally joined the bloc in 2013.
It is yet to join Schengen or the eurozone but in his 2017 State of the Union address, European Commission President Jean-Claude Juncker called on Croatia to be admitted to the former as soon as it meets all the relevant criteria.