I'd say Quality of Life mods, recipe mods and so on, technically 1.12.2 is the best as it has a lot more mods to use but if I were to stay with 1.14 Forge I'd say JEI for recipes and cheats, Xaero's Minimap or VoxelMap to show how different using the Vanilla map system and Minimap mods can be while why many people rely on Minimap mods XD.
If I were to go for mods with more building materials or technology or adventure/RPG elements I'm not sure really. Many are still in the works for 1.14 while others are fairly supported or ready. For a dungeon/adventure one I'd say Dungeon Crawl (which is inspired by Roguelike Dungeons a long time Vanilla with many tweaked aspects dungeon mod with cooridors of mobs and loot and many floors to adjust to). For tools expansion with materials and parts and upgradesI'd say Silent's Gems and Silent Gear (many would say Tinkers but as it's not for Forge 1.14 yet Silent mods are just as good although slightly tool and magic rather than Tinker's tool and tech approach).
I'd say take a look on Curseforge for any you want to try out (even check my signature below, the 'Fabric Discussion Section' as I post Forge mods there too besides being a Fabric wiki it is a 1.14 mod posting hub every day) and see what ones are interesting to you, just keep in mind many are a WIP in terms of porting over their content while other smaller ones or ones that have been around for a while in 1.14 are most likely ready. Fabric mods are more stable as the modloader has been around longer since the snapshots even while Forge 1.14 (as well as 1.13 when changes happened for it and they just moved it to 1.14 to fix the rest up) is in Beta but just be careful what you use.
-Just Enough Items (A recipe mod, trust me many people use it since you won't know the recipes from wikis or a recipe book so you'll need this mod, although with my mod experimenting it's not worked out for me in 1.14 but it depends, you might not have issues like I have with it)
-Silent's Gems (the materials mod, it's separated to allow for more options and probably work for compatibility) (needs a core mod called Silent Lib, get used to some big mods having core mods, libraries or with tech mods Power APIs)
I'd say that's enough for an idea of what mods exist, don't go too over board though and remember 1.14 Forge isn't 100% stable yet but I'd say experiment with many mods that feel Vanilla like the furnaces, chests, and so on, then go to upgradeable tools like Silent's or Tech mods or whatever else that deliver a lot of content and many times overhaul the game like it's a completely other game or massive DLC for the game.
I have a lot of videos trying to cover Forge mods too on my YouTube channel if you want also (not trying to plug myself but for the ones I do know work and are stable besides many I picked above that many I know are stable and many I know are just great but I haven't covered as they are more mainstream and I cover mainly newly uploaded mods for the week or niche mods, but with these videos you can tell the ones I'm familiar with that do work)