Sicario2 - Tier 1 probably the EL final against Chelsea. That being said it became very clear how broken the squad (and the club) were, which mitigated for how tough the early Arteta years were. Since then, we've been challenging for the league, which is more than you can say for the roughly a decade prior to Arteta joining. That makes a difference for me.
I guess at the heart of this is that I'm proud of who we are as a team and a club these days (versus my frustration with the second half of the Wenger years). And everyone currently at the club has played a massive part towards that. So even when there are missteps, I don't completely lose faith in the direction of travel. Doesn't mean I don't want more.
Not wanting to sound preachy but there's enough going on in the world to expend your emotional energy on. Something I've personally worked on is trying to react to things that happen in my life proportionately. Consciously or unconsciously, I don't invest as much of myself in the fortunes of the club as I used to. Doesn't mean I don't care, just means it's easier to be disappointed, accept I'm disappointed and move on.