I loved me some Overeem when I was younger, but honestly he wasn't all that as a Kickboxer either.
Kinda got lucky with how the Grand prix played out, when he won it.
He had his best success as a striker on the horsemeat, his style was mostly a high guard and trying to get in close for knees or the clinch in general. If we go purely by Technique I would say a Pedro Rizzo had better technique, Kharitonov and well the most obvious choice Semmy Schilt. Schilt was not the most spectacular, aside from his size obviously, but he played to his advantages extremely well.
Alistair might be one of the HW with the best degree in striking because of the GP he won (Like I mentioned kind of got lucky there) but I wouldn't say he was the most technical striker.
Ecoreem did work on his striking game well but overall Alistair always was more pure strength than finesse.