The usual advice is to pick a bull that produces the lowest inbreeding in the potential calves and has had the fewest progeny.
The number of progeny is more important for recently working bulls such as Viking and Fergus - as their direct progeny will be still living. Adonis was born in 1980 (I think), so although he's had more progeny, they've been spread over a much longer period and will have a less concentrated effect on the current national herd.
Does it have to be the same bull for each? - you could go for Adonis and his old genes and either Viking for his nature and colour or Fergus for his nature and his black feet. There's some theory that black feet are less trouble and less likely to need trimming - not sure about that one.
Sue