Doom1 vs Doom2: which one is the best to base my mod on?

I'm starting with Doom modding and i'm attempting a multiplayer-focused mod. Should i use doom1.wad as a base for my mod or doom2.wad? Can i use the shareware versions?

Also, for total conversions, what would be the best option?

Doom 2 is the best for modding and tc's.

Either one can be used as a proper base as evidenced by some great pwads published in the past.
However, for a beginner DOOM2 may be a better choice.

Use Doom 2 if you want the added monster roster and such. DON'T use shareware!

More variety available in Doom II (more monsters, SSG, megasphere). Most new stuff for Doom is actually stuff for Doom II.

Loading addons is normally disabled when playing with the shareware episode, to incite people to buy the full games, so don't try basing your mod on the shareware version.

Lastly, can you elaborate on what you mean about basing? Please do not edit directly an IWAD. You can create a new empty PWAD and add content to it instead of taking the IWAD and changing its content. Modified IWADs are frowned upon and you won't be able to share them on these forums.

If you're limiting yourself to PrBoom+ and aiming to make at least an episode of levels, you may also consider Doom 1. Its episodic structure may be useful. You can then fill the gaps by importing monsters from realm667.com to replace the Doom 2 ones.

I've decided to use freedoom2. Works great so far! Thanks for the advice guys.

Be aware that the Freedoom(2) IWAD contains textures that the regular Doom(2) IWAD doesn't, and if you use those textures in your maps, your wad will only be playable with Freedoom(2) and not with regular Doom(2). Well, unless you reincluded those textures into your wad, resulting in both Doom(2) and Freedoom(2) resources in the same map if it was played with regular Doom(2) - but I don't think it'd be a preferable solution.