UEFA Champions League is the biggest competition in European club-level football. Some of the biggest names across the continent go head-to-head for the coveted European Champion Clubs' Cup trophy every season.While the title is up for grabs for all teams registered with UEFA, teams from Europe's top five leagues have dominated the competition. Since Jose Mourinho's Porto in the 2003-04 season, no team outside of Europe's top five leagues has managed to emerge as the winners.UEFA Champions League@ChampionsLeague🗓️ Round of 16 memories: HEROIC defending from @officialpepe @FCPorto | #UCL3:16 AM · Feb 9, 2022137484🗓️ Round of 16 memories: HEROIC defending from @officialpepe 🙌@FCPorto | #UCL https://t.co/2JeI2VClqUWhile winning the competition proves to be a mammoth task, making it into the knockout stage is comparatively easy for some of the big clubs.Champions League round of 16 has seen some great matchups in recent yearsRound of 16 is the first round of knockout stage fixtures in the Champions League, with the top two teams from each group qualifying for the two-legged fixtures. Two title favorites inevitably end up squaring off at this stage of the competition.The standout fixture in the round of 16 is the high-profile clash between Real Madrid and PSG this season. The games between Inter Milan and Liverpool; Atletico Madrid and Manchester United should also make for interesting watches.As the Champions League action returns in mid-February, here we take a look at the five players with the highest number of appearances round of 16 fixtures. (Appearances counting the two legs separately)#5 Sergio Ramos - 25 appearancesSergio Ramos will return to Real Madrid for this first time since joining PSGSergio Ramos is one of the most successful defenders in the history of the Champions League, winning it four times with Real Madrid. He left the Spanish giants last summer to join PSG, so it is almost poetic that his first appearance in the round of 16 of UCL is against his former club.Recurring injuries have prevented him from performing to the best of his abilities with the Ligue 1 club. Nonetheless, one thing we can be sure to expect from him is his passion for the game.He has never lost a final game in the competition. So if PSG win all their games in the knockout stage and keep him fit enough for the title-deciding game, they might finally win the elusive trophy, right? Well, Mauricio Pochettino wished it was this easy.#4 Andres Iniesta - 25 appearancesAndres Iniesta won two trebles with BarcelonaSpanish clubs have dominated European football in recent history and they have been the most successful sides in the Champions League. They have won the competition 18 times and finished runners-up 11 times, more than teams from any other country.So, spoiler alert, players representing La Liga clubs dominate our list. Barcelona icon Andres Iniesta lifted the continental trophy four times during his spell with his boyhood club. He played a key role in the treble-winning campaigns in 2008-09 and 2014-15.UEFA Champions League@ChampionsLeague#OTD in 2002, Andrés Iniesta made his Barça debut. The rest is history. @andresiniesta8 | #UCL10:59 AM · Oct 29, 2021123591307#OTD in 2002, Andrés Iniesta made his Barça debut. The rest is history. 🏆🏆🏆@andresiniesta8 | #UCL https://t.co/lUyw7MhLxwThe 37-year-old had vowed not to play against Barcelona and thus bid adieu to European football in 2018. He now plays in Japan with Vissel Kobe, featuring twice in the team of the season in the J-League.