I haven't played vanilla campaigns for a long time, nor even custom campaigns and Reforged campaign. I'm usually having a fun with melee games vs computer on different patches (1.24b, 1.26a, and 1.29.2), though at some occasion I even played DOTA maps vs computer and custom games (like TD or RPG).
As stated above by Retera, the Reforged campaign is very good to go, although it's prone to various bugs if you played this on HD asset mode (some of them were fixed if you were on the latest patch), so you could do what Retera suggested: playing on Classic HD asset mode if you don't risk bug. There's also a "harder" version of Reforged campaign (the one with 1.33 balance changes). It was originally scrapped in one of the patches, but it's now playable on the latest patch through custom campaign (thanks Kam).
For the Classic, it's very much complicated. If you, for whatever reason, want to play the game in its original iteration, clunky, buggy, and unbalanced, try playing campaigns on both ROC 1.00 and TFT 1.07 if you have an original disc. However, as Retera mentioned, there's a point where you must patch TFT to 1.14 for one reason: the Bonus Orc campaign. Although the missing 2 acts (Old Hatreds and Blaze of Glory) were added in 1.13, that version still has a serious bug, pertaining the slow attack speed for all heroes after completing Act 1. This leads me to the next point:
The best tradeoff in my opinion is playing on either 1.26a and 1.27b. These 2 versions have most of the modern features and are far more stable. You don't need an original disc to play, the Widescreen option is available (though stretched), and most people stayed on those versions for one reason: the ability for online play (pirated server still works, but not official) and LAN. Some people had even favored 1.27b only due to the map file size limit being patched with this version, particularly for multiplayer hosting (8MB to 128MB), although this version is prone to random crashes that 1.26a doesn't have.