So I decided to play Ultima Underworld, is there the best way to enjoy it ?
Like some recommended mod ? Should I use the dosbox version that gog included or a newer one?

The GoG DosBox version has worked fine for me, not that I've ever completed it.

play the GOG version with my UltimaHacks patch, which adds mouselook among other things, if you can figure out how to apply it:

https://github.com/JohnGlassmyer/UltimaHacks

someone wrote instructions here which might be helpful:

https://www.pcgamingwiki.com/wiki/Ultima_Underworld:_The_Stygian_Abyss

let us know if you need more help.

DOSBOX GOG version is fine. I've never really explored all of the bells and whistles of the newer versions of DOSbox. You may want to play with the *.conf file that GOG creates a bit to optimize some settings, particularly if you're going to be using the CM-32L rom for better sound FX. (or you may want to just use the Ultima Patcher https://www.pixsoriginadventures.co.uk/ultima-patcher-1-68/)

I highly, highly recommend installing the mouselook patch here: https://github.com/JohnGlassmyer/UltimaHacks#ultima-underworld-the-stygian-abyss

It allows you to use your mouse to look around, which is turned on and off with a toggle switch (because sometimes you'll need to use your mouse to grab items and do other things), and also adds a bunch of really useful hotkeys. The manner to install it with GOG is a bit clunky, but can be figured out if you read the instructions very slowly and methodically attack each step. The most complicated part is re-creating the iso image of the game.gog file and copying it back into the directory.

When you unarchive the game.gog file, you'll notice that it contains both Underworld 1 and Underworld 2, so you might as well patch both games, then create the iso image, and then copy that new game.gog file into both your Underworld 1 directory and the Underworld 2 directory.

Happy gaming, and feel free to ask any questions on here if you need any clarifications.

Hi sorry for late response I've been busy. Anyway I was able to patch the game and setup mt32. Now do you have any recommended settings for graphic mode, scaling engine and window resolution?

The original resolution was 320x200, this is a 1.6 pixel aspect ratio (equivalent to 16:10). So don't use the 4:3 aspect ratio, the inventory slots should look circular.