Depends what you're looking for?
ES2, Stellaris and GC3 all have their merits and pitfalls.
ES2 is unbalanced, buggy and AI is still fairly average but it has its unique races and playstyles so is entertaining...when it works. Has a fantastic UI and art style as well but needs more polish/balancing before I could recommend it. It's definitely a good base and I fully expect amplitude to improve it going forward.
Stellaris is more grand strategy than 4x in all honesty, akin to other paradox titles. It's also real time and not turn based. Combat is a bit rubbish and essentially boils down to whether you have the bigger more powerful death stack. That being said the latest expansion Utopia is excellent and Stellaris overall does a lot of things well. If you can pick up Stellaris + Leviathans + Utopia and aren't too fussed about the combat side or the fact it isn't turn based, then I'd suggest that is probably my favourite.
GC3 vanilla was poor..micro management and Starbase spam nightmare also lacking content. DLC (which there is a lot of) and Crusade has improved that but tbh despite the epic ship creator I just don't find myself going back to it much.