When I feel like Doom is getting repetitive, I change to an another WAD or I just play a mod or a TC. There are also times when I switch to a completely different game like Super Mario Bros. or Pokemon for example
I play through the official IWADs very often, especially Doom 1 and Doom 2. Sometimes I want to play mods I recently downloaded, I don't bother looking for a custom mapset and some others just to have fun for some minutes or one hour
I may like the BFG and hate the Archviles, but I'll choose the BFG to go. With the Archviles you can make great combat scenarios and nasty tricks like those from Plutonia for example. About the BFG, eh I don't like using it that much. I prefer using the Rocket Launcher, the SSG and the Plasma Rifle more than the BFG.
GZDoom is an awesome source port, it can run most WADs and there are many cool+fun mods made for it. But it has problems with perfomance in slaughtermaps (Nuts.wad for instance), so I recommend you to use PrBoom+ for those type of WADs
I have played many good MegaWADs. I would recommend you to try Alien Vendetta, Scythe, Hellbound, Eviternity, Going Down and Japanese Community Project. They contain challenging, fantastic and fun maps to play :)