Hellas Verona winning Serie A in 1984/85. For me, it's a more impressive achievement than Leicester's. It was coincidently the only season where refs were appointed randomly, after Verona's surprising win that rule was quickly redacted. Just take a look at their competition:
- Juventus had won the European Cup that season and had a team with Michel Platini (current Ballon d'Or winner with 3 in a row), Zbigniew Boniek, Paolo Rossi (another Ballon d'Or winner), Marco Tardelli and one of the toughest defensive units of all time (Scirea, Cabrini, Brio)
- AC Milan had 3/4 of the best defensive unit ever assembled on a club level — Baresi, Tassotti and a young prodigy by the name of Paolo Maldini had made his debut that season (okay, Maldini is a stretch since he was far from being a regular, but still)
- Fiorentina had Passarella (World Cup winning captain and the 2nd best South American defender of all-time), Gentile and legendary Sócrates
- AS Roma had Falcão and Cerezo — one half of one of the best midfields ever assembled, Brazil 1982 (Sócrates & Zico, the other half, also played in Serie A at the time), Bruno Conti — one of the best players of current World Champions, and a bunch of quality players like Pruzzo, Graziani, Ancelotti, Nela & Di Livio
- Inter had Rummenigge (twice Ballon d'Or winner), Bergomi, Zenga, Causio, Brady (one of the greatest ever players from Ireland), Ferri... Zenga, Bergomi and Ferri, combined with AC Milan's Baresi and Maldini, would create the greatest defense in international football's history (Italy in 1990 WC)
- Sampdoria had Mancini, Vialli and Vierchowod — the spine of their league-winning team from 1990/91 (they would also reach a European Cup final and won CWC). Also, they had Souness
- Udinese had Zico, arguably the greatest player of the late 70's/early 80's (his only competitors — Platini & Rummenigge, also are playing in the same league)
- Napoli had Maradona, currently the most expensive footballer in the world & Ciro Ferrara
- Torino had Júnior
- Lazio had Michael Laudrup
Basically, only a couple of teams didn't have at least one world-class/borderline world-class player at their disposal (and many of them had multiple ones). Yet somehow, Hellas Verona with only 2 star players (Preben Elkjær & Hans-Peter Briegel, not the most household names, although, perhaps, unfairly), won the league.
Cagliari winning Serie A in 1969/70. This is their first and only title — Cagliari was a small team from Sardinia that only got promoted from Serie B a few seasons back. They've conceded a mind-boggling 11 goals in 30 games. It's a similarly miraculous story, but the league was considerably weaker.