Seravy2: "Master of Magic" - This is the old game from 199x with all of its original bugs. Avoid unless you like it when a game crashes to desktop or worse. The link in the above post shows...the first half of the list of bugs that were fixed by...
"Community Patch" - This fixes bugs. It does not make balance changes (except when fixing a bug wasn't otherwise possible). Basically, same as above except it works. It might FEEL different though because the AI is actually able to play the game here instead of getting stuck in a bugged state most of the time. It's not necessary very good at it, but the bugs (or missing features) like "AI doesn't see their units so cannot attack" or "AI can't attack on sea" really make a big difference. That link is to the 1.50 chances, which did have some balance related contents, but all of these were removed in 1.52 when the patch was made official.
"Caster of Magic" - This makes balance changes. Lots of them. Also, it goes way beyond fixing bugs in improving the AI so it's a lot more difficult.
If you haven't played either before then it really depends on what kind of experience you want.
If you like unbalanced games with lots of very powerful, gamebreaking (and just as many obviously useless) spells, units and strategies, then the Community Patch. I know that doesn't sound so great but honestly, it is a lot of fun to discover those gamebreaking strategies the first time. That's why the game is still alive 26 years later. It's just that winning that way did get boring after a few hundred games played at least for me.
If you prefer something that requires a bit more thinking, has a better pacing, no useless or unbeatable elements, still has many very powerful strategies but nothing that's outright unbeatable, and a better but sometimes merciless AI, then Caster of Magic.
If you are curious how a game with 1000+ bugs feels like, then you can try the original without the Community Patch - it was a fun game even despite the bugs - but I can't really recommend that, the patch exists for a reason.
You might want to identify your connection to the game when giving advice like this.