I totally agree with the T205 comments as well.
McGraw, Matty and Cobb have to be at the top of the easiest.
Also E90 Wallace is a good call.
Some of the caramel issues are a bit harder to put a finger on the easiest card. E94 are all so tough I couldn't really say.
I'm tempted to say that for E98, Dooin is the card that I've seen the most of and was the easiest to accumulate the 4 colors, but usually it's the big Hofers that have survived in the largest numbers so it's hard to say.
To immediately contradict that statement,
, there are a couple caramel sets where the hofers/stars are the hardest to find. E90-3 with Chance, Gandil come to mind.
E90-3 Schulte is in my opinion the easiest to find from that set.