For the DLC the race packs are usually pretty good with the Beastmen being the most enjoyable, Norsca and Chaos Warriors are enjoyable as well while Wood Elves are okay but I find there victory condition to just be a pain to do especially as they are limited with what they can build outside of there starting area.
The Grim and the Grave and The King and the Warlord are nice additions as well adding a new legendary lord and some units to the 4 main factions but not essential.
For mods the primary ones I use are Radious Mod and Radious Unit mod that adds a whole bunch of new units to all the factions and makes the AI more aggressive and build better armies making them more of a threat and a challenge. Conquer Anywhere because it just didn't make sense that things like Orcs can't take over Human settlements because they do... granted they are more fancy looking war camps but still. The rest I have are minor ones I just use for fluff reasons like Legendary Lord Unique Starting Positions that puts each of the lords where they are actually based in the lore e.g. with Orcs Grimgor stays where he is but Azhag moves all the way up north next to Kislev as that is where he raided until he found his fancy hat.