I'd say top AC without focusing on it would be about 21. Full plate, large shield and maybe +1 dex. If you are trying to get high AC but aren't trying to max out the character (no racial or class related tricks, or Pun Pun etc) One could have an easy 24 on a small character, if you use tower instead of large shield.

it is interesting, that AC at low level can easily be near 20, making the armored characters near impossible to hit, with 1st level encounters having attack bonuses probably +3 or lower.