As of "the most unexpected team mate battle results" I mean cases in which the driver who had been expected to be beaten turned out to be actually faster, or the driver who was expected to be totally dominated turned out to be really close. Being higher in WDC only thanks to luck is not what I'm looking for.
The ones I can think of:
1. Ricciardo vs Vettel (2014)
Vettel had just won 4 consecutive WDCs, dominated Webber since 2011 and had had a superb season in midfield Toro Rosso in 2008, while Ricciardo looked just OK in Toro Rosso but few people saw him as the next big thing and some even questioned if he was really that much better than Vergne and deserved to be given a seat in a top team. Despite all of that, the Australian driver came out ahead by quite a margin.
2. Hamilton vs Alonso (2007)
While Hamilton was widely acknowledged as a great talent, Alonso was widely seen as the best driver on the grid. Even if it was not a shocking opinion to see Lewis as equally talented driver to Fernando, everybody was aware based on the past history that even the most talented drivers were rarely able to perform on a WDC worthy level since year 1 in F1 and it took a decent amount of experience to get to anywhere near one's peak. Hamilton's case proved to be different as he started matching Alonso just after a few races and even beat him in the final WDC standings.
3. Norris vs Ricciardo (2021)
It's kind of funny to see Daniel featured here in both roles but it's how it's. His record against his team mates had been very good, as he was significantly faster than all of his former team mates bar Max, and that list includes a multiple WDC like Vettel but also exciting talents like Ocon. He was also much closer to Verstapen than any other driver since, while all Norris could show on his F1 resume were 2 seasons lost against Sainz, whose own record against his team mates was not that stellar either, including a loss to Nico Hulkenberg. Ricciardo was definitely a favourite to win this confrontation, maybe not as overwhelming one as one could deduce from my previous lines, as Lando enjoyed a lot of support and opinion of a talented driver. There might have been some people who called Norris as the winner of this battle but the extent of his advantage over Ricciardo in the first half of the season must have surprised every neutral observer.
4. Rosberg vs Schumacher (2010-2012)
This one can be a bit of a controversial inclusion since with hindsight it totally made sense that Schumacher wasn't at his best after the return and Rosberg's estimation went quite a bit up between 2010 and 2016 but since the topic is based on expectations from that time period, I think this battle deserves to be here. I remember many people didn't really rate Nico at the time and he was supposed to be an easy target for even a past-peak Michael, at the very worst it was supposed to take a few months for him to start beating Nico on a consistent basis. As the evidence I can bring the data from "Place The Points Competition" from this forum where out of 34 players, 28 expected Schumacher to end up the year with more points than Rosberg. So yeah, don't let the hindsight trick you, Schumacher was overwhelmingly favoured to win this battle but he didn't manage to do it.
5. Massa vs Raikkonen (2007-2009)
Raikkonen had been seen by many at the time as the fastest driver in F1, while few people had really expected Massa to ever become a proper top driver, yet the pair turned out to be closely matched. Even 2007 was closer than it may seen, Felipe was leading Kimi in the standings after 12 races but the mechanical DNF at Monza made Ferrari to turn its focus towards Raikkonen as their chances of chatching up to McLaren's drivers in WDC grew smaller and smaller.
Some of these weren't exactly sensational but it's the best I can think of. My memory can only go as far in time as 2006, so perhaps some of you will be able to provide examples that will vastly enrich this list.