I've found red to be the toughest color but I have seen more reds bouncing around lately. I've put the set together 4 times card by card and sold them off to buy other cards but I have kept my last set and am very close to an all red subset. The red Lajoie is really tough for some reason and that is still the one card I regret selling and one I need.
As far as order of scarcity, a few more Vaughns and Coombs have appeared lately but I think those are still the two toughest commons, followed by Dahlen and Kling. Chase is not easy but I think he has been hoarded and his availability is influenced more by this fact (popularity) than true scarcity.
As far as the Hall of Famers go, Walsh and Mack are the toughest by far. Wagners are being hoarded (again because of popularity) but Ed and Connie are very hard to find. Interestingly, most Walshs which appear have red backgrounds, directly contrary to the color scarcity tables. The orange Walsh is the toughest to find for some reason. As far as Mack goes, the pose and popularity make this card overpriced (IMO) and tough. I have been searching for a red Mack for some time but usually I only find blues and greens and those in lower grade.
A master set of E98s is doable. Obviously there are 4 Cobbs, 4 Wagners, 4 Youngs, 4 Macks, 4 Lajoies, 4 Vaughns, 4 Coombs', etc. but it is doable if you have the money and don't mind upgrading along the way, unlike the master set of E94s which has cards which rarely if ever surface in particular colors.
