Can't bring to mind a good book or paper that compares all the countries side by side, but still fairly sure UK would top the list.

For a good introduction to the sad state of UK elections:

> The electoral process is "rigged". In 2005, if you total all the votes cast for the main parties, it took an average 26,906 votes to elect a Labour MP, 44,373 to elect a Tory MP and 96,539 votes to elect a Lib Dem MP. Not since the rotten boroughs of the 18th century have elections been so corrupt.

https://www.theguardian.com/commentisfree/2010/may/03/democr...

> All votes should be worth the same, right? With First Past the Post votes are not equal. In the last election it took 26,000 votes for the SNP to win a seat compared with over 800,000 for the Green Party.

https://www.makevotesmatter.org.uk/first-past-the-post

Looking at any recent election will show you the discrepancy quite clearly - the difference they are referring to is the difference between popular vote percentages (nationwide) and number of MPs for a given party (FPTP split by constituency). Parties with high popular vote percentages often get very few to no representatives because they were never the top candidate in any constituency.