Bartali, Coppi, Anquetil, Merckx & Hinault are the only ones who should be on the short list.
Numerically, Merckx has to take it, but Coppi had a world war interrupt his career so we'll never know. Hinault isn't far behind Merckx in what he won but loses out on sheer volume.
The rest - They all have some sort of shortcoming. Either a lack of GT's ot Monuments, or they are too one-dimensional.
Lemond only won the Tour and as Hitch said no classics. Kelly was predominanly a classics man, points winner & shorter stage race winner. His Vuelta is from when a sprinter could win it. Indurain & Armstrong were only really interested in the Tour, with anything else being a bonus. Gimondi was unfortunate to turn pro in the same year as Merckx, hence his less full palmares.
If you stick with 10 to choose from there are a few that should be in over some of the five here. Maertens & De Vlaeminck come to mind immediately.