Max HP at 1st level, rolled at levels 2+...but I allow a single re-roll if you roll a "1".
BTW, you may be interested in how the RPGA campaigns do it. You get max HP at 1st level (and, at 2nd, in at least one campaign)...then, at levels 2+, you get a fixed amount.
That fixed amount varies by the campaign:
- In Living Greyhawk, it's (1/2 max)+1, so a barbarian (d12) would get 7, a rogue (d6) would get 4, etc. (you add the Con bonus after that).
- In Living Arcanis, which is known to be a tougher campaign, you get max HP at levels 1-3, then 75% (round down) of max at levels 4+, so a barbarian would get 9, a rogue would get 4, etc. (and, again, add the Con bonus to that).