Hamilton - Lewis obviously has plenty of brilliant seasons under his belt, and his clean 2015 deserves a mention too, but I feel his 2017 with a difficult Mercedes is criminally underrated. Lewis went wheel to wheel with his main rival Vettel three or four times, and came out the winner each one of them. Masterful at avoiding incidents, I feel he wasn't as complete a driver in 2012 or 2010.
Bottas - For me his 2014 pips 2016 and 2017. His mid-season cool-headedness and podium streak was something to behold.
Vettel - 2011 and 2015 deserve a honourable mention, but Seb's 2013 was absolutely relentless. Say whatever you like about Newey-mobiles and blah blah, but people forget Webber was absolutely nowhere near Seb that year.
Raikkonen - 2005 slightly pips out 2003 and 2007 because of his team-mate; especially in quali-trim, Kimi was the scary monster I was expecting JPM to be. Who can forget those quali laps at Monaco and Monza?
Ricciardo - it's often overlooked how good his 2016 was, but 2014 he was a revelation. Whenever Mercedes dropped the ball, Danny Ric was there to collect it. Not to mention his exciting overtaking form.
Verstappen - 2017 was ridden with unluck, yet it didn't impact Max's form and he truly got rewarded end-season, remembering especially the great drive at Sepang.
Hulkenberg - the German's got a tendency to put together brilliant half-seasons and be a bit inconsistent on the other half; end of 2012 and 2013, start of 2014, end of 2016, versus the less impressive other halves... But I would rate Nico's 2017 the best overall: entering the year, I think Renault was complete shambles (as demonstrated by Palmer's struggles) and it looked like it was the Hulk himself that was carrying the team from the bottom of the midfield back to the top of it.
Sainz - been relatively consistent over his career really, but for me 2016 would stand out of the bunch
Grosjean - Romain has always been an up and down driver depending on his brakes - sometimes producing amazing drives into top places with cars that don't belong, sometimes looking like a complete idiot. 2015 was probably the season with the biggest ratio for the former instead of the latter, with the Spa podium as a particular highlight.
Magnussen - similarly to his team-mate, Kevin is a bit of an up-and-down driver, sometimes truly struggling with a Jolyon Palmer. But his 2018 has been more of the former.
Ocon - he's beating Perez with more of a regularity than last year. So, 2018.
Perez - hard to choose between the surprisingly mature, high-peaking 2012, and the clean and consistent 2016 that saw his team-mate Hulk leave Force India with his stock well lower than it had been - but I would lean for the latter if pressed.
Alonso - I'm not as convinced about 2012 as everybody else seems to be (for me it had something to do with Massa being in the wrong place mentally as well); for me Fred's 2006 was even more impressive; going into a true one on one battle with the beast still relatively fresh-faced, and coming out as the winner. But you still have to honourably mention his giant-punching 2008 and 2009 with terrible cars, 2013 and 2014, 2016...
Vandoorne - I mean, 2017 was not that bad. About what you'd have expected from a rookie entering a McLaren team, with Alonso having had a bit of time to build it around himself.
Gasly and Hartley - well duhh... They are both driving better in 2018 than last year, no?
Ericsson - has looked like he gets a little bit better every year, up until around Baku this year that is. So I'm going with 2017 and potentially ending Wehrlein's prospective F1 career by looking essentially not significantly weaker than him.
Stroll - had streaks of form in 2017 where he looked like he truly belongs, especially at the end of the European tour. However, when he couldn't find the pace then he truly could not find the pace - hence, it's hard to assess whether him not performing this year is entirely due to the car, or due to him and Sirotkin being unable to hook it up without the experience of Felipe...