The World Cup — I'm tempted to name one of Italy-34 or Argentina-78 due to fair play concerns... but I've obviously haven't seen any games from the 30's and that Argentina team was very strong with amazing performances from Passarella & Kempes. Or West Germany-54 due to, again, very strong doping allegations. If we're talking purely about on-the-field performance from fully televised World Cups... probably
Argentina-22. Then there's the outside shout for Brazil-02 simply due to how weak that overall tournament was. The only great team that they've faced in the entire run-up was England, but they did well to convincingly beat the opposition, regardless of how relatively weak it was.
The Euros —
Portugal-16 for me. Shouldn't have even made it out of the group if not for the ridiculous intumescence of the tournament. Finishing third behind Hungary and Iceland... barely scraping past Poland via penalties, facing Wales in the semi-finals and, credit where credit is due, playing decent enough against underperforming France in the final and beating them in the overtime with a lucky goal. There were lesser names that have won it but Portugal were the worst-performing winners, there's no question about it in my mind.
The Champions League — maybe
Chelsea-12? They have managed to beat great Barcelona & Bayern sides, so it wasn't a case of a lucky draw, but I struggle to think of a less convincing European champion ever. I don't know how many innocent leprechauns were sacrificed on Abramovich's yacht for them to finally win it. The early 1980's European Cup campaigns were weird sometimes. I don't think that, say, Aston Villa or Hamburg SV were the best sides in Europe that seasons but they both had to beat a strong Dynamo Kyiv side and then a proper European heavy-weight in the final (Bayern & Juve respectively). Another shout would be
Porto-04, of course — they shouldn't have beaten us if the ref was competent (or, possibly, corrupt?
Apito Dourado) and the rest of their draws were stupidly easy. It still was a very strong team but then any team that wins EC/CL is.
The Premier League — it pains me to say, but...
Leicester-16. They're also the best champions due to their incredible story, but the league was at its absolute low with every big club going through some sort of a crisis, so if we go by performance, I wouldn't bet on them against any other PL-winning side. Kanté (especially), Mahrez & Vardy were genuinely great and the rest of the team went through the collective purple patch, but still.
La Liga — again the worst champion = one of the best stories? A very competitive 99/00 season with Barcelona, Madrid & Valencia sides that performed well in Europe around these years. But that
Deportivo-00 side still finished on 69 points (record low for a winner since they've introduced the 3 pts for a win rule) with a relatively low +22 goal difference. It was a very entertaining side to watch though with Makaay and Djalminha especially stealing the show. Who else would perform a rainbow flick against Real Madrid...