At the risk of alienating all the Golden and Lab people here, retrivers are friendly only in the sense that a subway fondler is friendly. Digging strangers out of my personal areas is not, in my mind, friendly. Intrusive and overly familiar, yes, but not friendly.
In my experience, spaniels are the friendliest types, overall.
Easiest to train - speed division: Collies of various sorts learn in 2 seconds, though you might want to specify whether it's important to you if they actually continue to pay attention to a word you say after that 'sit' command. They sit, then they take off at the speed of light. Quick, definitely quick dogs.
Easiest to train - reliablity division: probably the working and terrier groups are the best combo of being relatively interested in what you're saying, and being dependable about carrying out instructions. Collies hang onto every word, then pretend you said "run across that highway now, and take this pot roast with you as a snack."