TS4 is the only Sims game I play with any regularity now. I have tried to go back to Sims 2, and can't bring myself to play it for long, and I've played a bit of Sims 3 since, but can't get into that either. Full disclosure - I have all EPs, GPs and stuff packs except one stuff pack.
For me, TS2 was like a high-school sweetheart that I recall fondly with starry memories, but years later I try to go back and rekindle the romance and sadly it's just not there.
TS3 was like that solid but slightly dull romance where the lover showed me a good time and we did a lot of great things together, but there was just never that "spark."
TS4 is the extremely good-looking partner who is very easy to talk with, and we always get along well. However they started out kinda poor and were just taking me to the same coffee-shop over and over.
However over time this partner has tried earnestly to woo me over and started taking me to more and more interesting places and we've done a lot together now. Now and then they really surprise me. And they are just so easy on the eye.
Seriously though, if Sims 4 just had the following things it would be near-perfect for me: wants (better whims), fears, an attraction system, and TS2-style memories.