Sehwag is massively over-rated in the "great player" debate. He averaged sub 30 in NZ, England, SA. I really liked watching him bat but if India were batting away, there wasn't much batting to be seen. I remember once I hit the snooze button Australia away(5am IST is play) and he was gone by the time my alarm rang 2nd time.
Not even "opener", just in the batting pantheon I rate Cook very highly because for him to play at home in the conditions he did and make 10,000 runs is serious skill. The list of English openers of the last 20 years will have Cook with 160 matches, Strauss with 100 and a very long tail of batters with < 20 matches? Probably the one thing you can hold against Cook is I think he peaked through a period where bowling stocks were a bit down. There's been a resurgence of great fast bowling in the last 5-6 years or so, but it was a bit hit and miss before then. Cook played through a period when Aus had likes of Siddle, Hilfenhaus who were OK but won't be remembered.
Every country has its holy grail test tour and for England it's Australia, India, and Cook made lots of runs in both milestone series wins.
This is also why I think Kevin Pietersen was different class despite his relatively poor average for a middle order batter..