Well that depends. Generally not, but you also dont know what kind of armors you will find in the game. And I dident specify that plate is end game armor. Id say magical armors (of any kind) that gives the best AC and/or has other benefits could be the best one. AC isent the only factor involved in that. I never mentioned plate beeing end game armor so I dont know where you got that impression from?
That said if you are just looking at AC.... going for 20 DEX is at best possible at level 6. Which would mean that before that time the heavy armors would give a better AC. Infact considering you could have a DEX of 17 at the very highest at character creation chain mail would always give you better protection. At level 4 you could get an equal protection with an ASI or a feat that grants +1 dex (and some other benefit) and only at lvl 6 when the next improvement comes up would you get that AC of 17.
Considering its not hard to start with a AC of 19 at lvl 1 with chainmail armor, defense fighting style and a shield, im not really impressed with an AC of 17 at lvl 6 for races with +2 DEX.... Mind you, you start with chainmail in the pnp, not in BG3 so I will condede that point. Not only do you not start with them they arent readily available in BG3 as of this time. Thats a very big problem and Larian needs to fix that, but my point stands. An AC17 for a fighter is pretty damn mediocre. With the same fighting style and a shield you get AC20 with DEX20. 1 point more then a lvl 1 fighter can get. But you need lvl 6 (for the 2 ASI's) to reach that point. Not really a shining endorsement.... Heck, my Elven wizard with mage armor has AC16 at level 1. Dont want to rag on about this so much but an AC17 for a fighter is pretty damn low.
Now DEX fighters are obviously more then their AC. DEX aids in stealth, initiatiive, stealing etc. but I mentioned the strength requirement because if people dont take it into consideration and dump STR, they might run into some problems later down the line. The better armors need 15STR to use without penalties. Melee attacks (other then finesse weapons) use STR. Thrown weapons use STR. Things like grapple or freeing from bonds requires STR. You can sometimes use DEX instead but thats not always an option. It factors into your carry weight. How far you can toss things or lift things. How far you can jump etc etc etc
You can use weapons that use DEX rather then STR, you can use light armor to use your DEX bonus to your AC and you can use a bow iin place of thrown weapons. You will be stealthier and have a higher initiative on avarage as well. But STR is used for more then just combat, which was my only point...
