Best Moments of the Season
There are fewer greater feelings in F1 than winning your first race (I'm just guessing here, surprisingly I haven't won a race myself). The win in Russia was made even sweeter due to the pressure he was under at the end of the race. His other two wins were remarkably similar in that sense.
Worst Moments of the Season
There were a few races, particularly towards the end of the year, where Bottas simply wasn't on the pace. Perhaps his worst race was in Malaysia where he seriously struggled. Sebastian Vettel managed to beat him by 20 seconds despite the fact that Bottas started P5 and Vettel started P20.
Target for Next Season
Bottas stayed in touching distance of Vettel and Hamilton for the first half of the year, he needs to make sure he's more consistent and has fewer 'off' weekends.
Whilst we can argue about Bottas's pace and whether he should've been quicker across the year, one thing that can't be argued is the fact that Bottas succeeded in his job. Bottas scored enough points to comfortably hand Mercedes the Constructors' Championship and he ended up being the perfect teammate to Lewis Hamilton as there were no bust-ups over the whole year. Say what you want about the Finn, Mercedes won everything they could have and they did it in a much smoother way than previous years.
If Bottas wants more personal success, he is going to have to step up his game. Beating Lewis Hamilton is difficult, but not impossible (see 2016). It's all about consistency and keeping up with him even when he's in somewhat of a purple patch. Bottas will be racing for Mercedes in 2018, but once again it's just a one year deal, he needs to do something to give management greater belief in him, or he may face the axe for a younger individual.