who had the best career overall?

I’d say Selleck over Danson by a half-length to 3/4 of a length. Guttenburg an 8th of a mile behind.

In order of the most acting credits it would be 1. Guttenberg 2. Danson 3. Selleck

But I'd go with Danson mostly due to Cheers which is one of the best sitcoms of all time. And he's still acting doing shows like A Good Place which are very popular. Selleck still has Blue Bloods which is popular and has had a great career. Guttenberg has enjoyable movies but I don't think anything jumps out as being like a legit great movie. They're like fun movies, comedies, but not all-time greats.

Probably Danson, with one of the most successful sitcoms of all time, another successful one, and continuing projects to date. Selleck had a megahit with Magnum, but that was forty years ago. He doesn't recite the approved ideology like all Hollywood pods are supposed to, so he hasn't got the work he has deserved. Now he plays a humorless wooden Indian on some cop show nobody watches. Guttenberg? No idea what ever happened to that dude.

1. Selleck, 2. Danson, 3. Guttenberg.

Danson's career had the highest peak, but Selleck has constantly had things going on for 40 years.