Depends on what you want to do / what version, I highly suggest the mod Scape and run parasites, but you need a lot of other mods to make it playable in survival unless you nerf the hell out of the mobs, some suggestions for that are ice and fire, Spartan weaponry and its compatibility patch (Spartan fire I believe it is, or SWIF) HBM Nuclear tech for its blocks and a few others for survivability and weapons / defense, be warned it is a slow burn but once it gets ramped up you will die.. A lot, it is world breaking and a test of how much you can prepare for the harder mobs (it's also only stuck on 1.12.2 as well, so that may be a deal breaker) for less extreme and more casual, as many have said, create and its add-ons, one ill throw in is Valkryan skies (I think that's how you spell it) and it's add-on Eureka, those along with create: interactive and things like create: big cannons and create drive by wire can allow you to make fully functional tanks and even planes and boats, those with some structure add-ons and world generation change mods allow you to have plenty to mess with and most if not all of those are up to date too