Hi Laure,
Although I did participate in the poll you created, I wanted to voice out my opinion on this (which would automatically have the implication of revealing what I voted for. No secret ballot here).
I understand Johan Hakkesteegt's point about 'I Was Right' sounding a bit arrogant but given the fact that in the grand scheme of things (in other words, the myriad other missions there are and there are going to be), I don't think it could be exploited by anyone. After all, and if my understanding is correct, you get this badge for just getting one answer right. So, if a user has got one answer right and joins the league of hundreds of others who are already there, I don't think they would really want to show off that badge. Plus, in terms of reputation, if someone really wanted to brag, I guess 'tooting one's own horn' riding the lifetime points rather than the badge would make a stronger case for the bragger. 
Besides, I share the opinion that 'I was right' sounds assertive (which is good) and serves the purpose of introducing badges (which I believe is to encourage user engagement) better than a less assertive badge.
Regards,
Akshat