If pace of play was not an issue (i.e., a private club that sees little play) would you answer differently?
Right, I think that is the most important determination; is it public or private?
My experiences at public courses, especially crowded ones, has led me to form the opinion that the par 4 is the ideal starting hole, simply because play has a tendency to back up on the par 3 holes and par 5 holes.
Often, on the par 5s, the weaker player, regardless of ability, will flail away with driver, and wait for the green to clear 350 yards away so they can mess up a bunch more shots, take their 8 or 9, and go to the next tee--all of this while other groups back up on the tee, often as much as four groups deep.
Par 3s, same thing. Many weaker players miss the green, dawdle around for awhile, and then hole out, without letting faster groups hit up and play through.
Par 4s can have similar problems, I know of one I used to play often that was the second hole, featuring a crossbunker 20 yds short--players would always lay up before the crossbunker even though it's only a 400 yard hole.
Private clubs, on the other hand, have less overall play, and might space tee times out a bit more--say, ten minute intervals. Plus, I think the quality of golf at most--I repeat most--private clubs may be better, so you may have less hackers and weaker players may be encouraged by staff to hurry along or pick up.
One of my favorite courses in the world--Bethpage Red--has a whopper of an opener, 471 yds, dead straight, but an uphill, blind green. It's a difficult hole, but not impossible with a good drive or a good wedge game. My key is to pick my line of approach from the tee--find the tree the flag is directly in front of--and aim for that on my approach.
Sometimes it comes off, sometimes it doesn't. I'm an 11.
I don't mind it because for my handicap, I can expect to make some bogeys during the round. It's a strong opener, although for public play, I don't know if it's a good opener. Play can back up here too, because of the length involved; the average player, hitting a 230 yard drive, will have 240, uphill, which usually means they hit a firway metal into the face of the hill which the green sits higher up on.
To continue the question, for public play, I prefer a reasonably easy starting hole--to ease the nerves and first-tee jitters, and seve as a more gentle introduction into the round.
I would not be opposed to two strategies at work, however; perhaps a way for the better player and the weaker. I like Ross' idea of a 'handshake' opener, a gentle introduction to the round, to get things going. A par 5 is ok as a second hole, but not a par 3, only because it causes backups. 4-4 is a good introduction, as is 4-5.