my 2 half-orc fighters have 18dex and it seems that heavy armor is useless to them as the armor class stays the same. what type of armor is best for them?

ps: what ac has a monk at lvl 12 with 18dex 18 wis ??

A fighter with a high DEX can get away with wearing leather or studded leather, because the max DEX allowed stat for leather/studded usually lets you use your entire DEX-based AC bonus. OTOH heavy armor usually has a max DEX bonus or +1 or 0 which means that that 18 DEX ends of being of less value.

This is one of the places there the 2nd and 3rd ed rules are quite different. In these 3rd ed rules, your tanks don't really need a high DEX. A DEX of 12 will be more than enough for a plate wearing tank. A high DEX fighter wearing studded leather can end up with a similar AC as a mid/low DEX fighter wearing plate.

<font color=skyblue>All of my characters have high Dexterity levels, and I have them all in nothing larger than light armor...like Studded Leather. It hurts my Fighter/Mage by 15% spell failure...but she still has a good sword arm. I personally like to play with faster-moving characters instead of men suited up in heavy metallic suits. That's just my gaming style. I never use sheilds, either. So, if you want to take advantage of that Dex...stay in light grade leather tunics.</font>