All so subjective, it's hard to decide.
First, 'the Ashes' is such an integral part of cricket it seems pointless to vote. It's like voting on whether 'hitting sixes' is great or not.
Second, I've decided I'm only going to vote on players, not innings or matches or overs or whatever.
Regarding players, everyone will have their own standards: for me, there are 'legends' and 'all-time greats' and 'great players', and they all have different criteria. For example, I don't think Boycott is an all-time great, but he might qualify as a legend -- in fifty or a hundred years time, he'll be remembered where greater contemporaries aren't.
To get my vote, the individual has of course to show greatness, but must also have the promise to be an all-time great, which requires that little extra. For active players, this means that although they could get there, if they haven't gotten there yet they don't get a vote. Thus in the last round of voting, I voted for Lara but not Tendulkar. Lara has twice broken the record score and will be remembered as long as cricket is played. Tendulkar is one of the best batsmen of his generation. There is a difference. My final criterion is that I don't rate one-day cricket at all, so people whose reputation is enhanced by ODI cricket lose out.
So, as to this group:
Imran Khan --- Yes
G.D.McGrath --- Yes
A.C.Gilchrist - No
S.R.Waugh - Yes
I.V.A.Richards - Yes
M.Marshall - Yes
D.Lillee - No
J.Kallis - No
McGrath is a yes and Lillee is a no, because Lillee was not the best bowler of his time -- Roberts, Holding etc. were better -- but there has never been a bowler who has been so consistently so much more deadly than his contemporaries for so long as McGrath.
Richards, Imran and Marshall are obvious, just as Kallis is obviously not (yet). If Kallis passes Lara's 400, in a winning cause in a meaningful match, I'll think again.
Gilchrist and Waugh are borderline: Gilchrist is not an all-time great wicketkeeper, and he is not an all-time great batsman; in combination, he might be, but I think his career is too short and perceptions rest too heavily on ODIs. Waugh is one of my favorite cricketers and his attitude, especially as captain, redefined test cricket. As a batsman, I think I'd vote no, but as a cricketer and for his influence on the game, he gets a yes.