This ended up a lot longer than anticipated.
I partially utilised
driver head-to-heads, the rest is intuition:
1. Alonso
Alonso has, in my opinion, been far and away the strongest driver on the grid every season since 2005, with the exception of 2007 where he was matched by Hamilton. I don't see why that would suddenly change now, so he is up there kind of by default. His advantage over Button has been quite large when the car hasn't broken down on him, which reinforces my placement.
2. Hamilton
Across 2013-2014 he and Rosberg were two of the closest matched drivers on the history of the sport. Now this season Hamilton is 9-1 up in qualifying and 7-3 up in races and is really making Rosberg look foolish. Although he lost a net 17 points to Rosberg with the pit stop nonsense in Monaco, Ricciardo's antics in Hungary lost Rosberg a net 16 points back to Hamilton.
This is surely one of Hamilton's strongest seasons yet, although his race in Hungary was atrocious.
3. Vettel
Vettel is miles ahead of Raikkonen in qualifying, about as far ahead as Alonso was last season (both just over half a second), and also holds a consistent advantage in races too. However, his race in Bahrain was rubbish, and that advantage he holds in races is significantly smaller than the advantage Alonso held over Raikkonen last season, with Vettel's mean advantage being ~18 seconds compared to ~33 seconds for Alonso.
Raikkonen currently has 47.5% of Vettel's points tally, but last season he held just 34% of Alonso's points tally overall and only 23.5% after Hungary. It has still been a very strong season for Vettel, and boy did he need it after 2014.
4. Kvyat
Despite his strong rookie season I didn't anticipate him being this close to Ricciardo, yet after 10 rounds the points tally is a very closely matched 51 - 45 in Ricciardo's favour. However, Kvyat is actually 4-3 up in races and if you exclude the points Ricciardo scored in Australia (where Kvyat didn't even make the start) then the points tally becomes a 45 - 43 in Kvyat's favour. Thus he is rated just above of Ricciardo.
5. Ricciardo
Although he holds a 6-4 advantage in qualifying over Kvyat and was very impressive in the opening rounds, Kvyat has come back at him in recent times to the point it is now Kvyat who has a small edge in races (as discussed above). Ricciardo has also been much scrappier and less consistent compared to last season and is still hopeless off the line.
6. Button
Solid as ever, pushed Alonso in the first few qualifying sessions, but has been consistently slower in race trim.
7. Rosberg
His race pace is much improved compared to last year, but his qualifying pace has vanished. He is only 21 points behind Hamilton, but only 21 points ahead of Vettel despite a significantly faster car. Too many mistakes.
8. Bottas
Has a small edge over Massa, leading him 5-4 in races and 77-74 in points despite not taking the start in Australia. But a small edge is not enough, Massa scored just 48% of Alonso points over their time together so if Bottas was the next big thing he should be sweeping the floor with Massa.
9. Massa
Although he is down in races and points relative to his team mate, Massa holds an impressive 6-3 advantage in qualifying (excluding Canada). Having to start from the pitlane in Bahrain and from 15th in Canada has hampered his points scoring potential, but he and Bottas are clearly extremely closely matched.
10. Raikkonen
Despite being humbled by Alonso and now Vettel I still consider Raikkonen to be one of the stronger drivers on the grid, with his poor performances against them serving as proof that the skill gap between even the top drivers is much larger than many fans and pundits would like to think. A couple of mechanical DNF's have also exaggerated the points gap between him and Vettel.
11. Grosjean
Was surprisingly matched 5-5 by Maldonado in races last year, but is a long way ahead this year.
12. Hulkenberg
Impressive qualifying pace, but a 4-4 race record vs. Perez is not good enough.
13. Nasr
Making Ericsson look foolish once again. Rookie of the year.
14. Perez
Decent, but unspectacular.
15. Sainz
Suffered badly at the hands of mechanical DNF's, but 9 points with what is allegedly the second best chassis on the grid is simply not good enough. In the 4 races where none of the 4 Red Bull drivers had a mechanical DNF, Ricciardo and Kvyat both accumulated 17 points whilst Sainz accumulated just 7 and Verstappen got 6.
16. Verstappen
Is behind Sainz 6-7 in terms of points scored per 'counting' race (I.e. races where neither had a mechanical DNF), and has been impetuous at times. Only his second season of car racing though, so impressive in that sense.
17. Maldonado
His speed has completely deserted him, but his recklessness on track hasn't.
18. Ericsson
Why?
19. Stevens
As above.
20. Merhi
In the overweight Manor chassis his weight has been a huge handicap compared to the diminutive Stevens, and the car itself
looks terrifying to drive. How much impact this has had can only be speculated on, but even with that in mind Merhi's season has not been good.
I consider the Top 13 to be worthy of staying in F1 on merit, and Verstappen too considering how inexperienced he is. Perez and Sainz are ok, but their seats would be better served by any number of talented junior drivers, or by Vergne. The rest shouldn't be on the grid at all.