2. Barcelona - 1 loss in 12 games

Defending Spanish champions Barcelona looked on course to return to the Champions League final for the first time in 4 years, after beating English side Liverpool 3-0 in the first leg of the semifinals. However, the Catalan challenge unravelled in the return leg, as four unanswered goals by the hosts eliminated Barcelona from European contention for the season.
Barcelona started their 2019-20 Champions League campaign on an underwhelming note, drawing goalless away at Borussia Dortmund after the hosts missed a penalty. Either side of another goalless draw, at home to Slavia Prague, Barca racked up four wins to reach the knockout round of the competition as one of 6 unbeaten teams.
The Catalan team's 8 European match wins is the most by any club to have played over 10 games this year, excluding qualifiers, behind only Chelsea and Liverpool's respective tallies of 10 and 9.
1. Chelsea - 1 loss in 15 games

Europa League winners Chelsea suffered their lone European defat this year in their first group-stage game of the 2019-20 UEFA Champions League at home to Valencia. The Blues racked up six consecutive wins in the Round of 32, Round of 16, and quarterfinals of the 2018-19 Europa League, before a penalty shootout win over Eintracht Frankfurt following two 1-1 draws in each leg of the semifinals booked an all-London final over Arsenal, which they won 4-1.
The Blues started their 2019-20 European season with a UEFA Super Cup outing against fellow-Premier League team Liverpool where they went down in a penalty shootout after ninety minutes of action produced a 2-2 stalemate.
Chelsea launched a remarkable fightback from three goals down in their group-stage game against Ajax, scoring three goals in the last half an hour to salvage a thrilling 4-4 draw. Qualification for the knockout round was assured following a 2-1 home win over Lille on Matchday 6, coupled with Ajax losing 1-0 at home to Valencia.
Chelsea's 10 wins (from 15 European games), and 7 clean sheets (also Barcelona) in 2019 are the most by any team to have played over 10 European club football games, excluding qualifiers, during the year.