For me,
TS1 is like my first toy, practically excited about it!. Pros, it's a practically a "new" & different simulation game. Cons, less gameplays.
TS2 is like my first crush, I think & play it every day, even before my big exams. Pros, practically an improved iteration, "drama with backstories", a lot of new gameplays, customisable lots & neighbourhoods', love the musics & audio, CARS & HELICOPTER :D. Cons, I can't think of anything :p
TS3 is like my first job, need it, love it, hate it. Pros, open-world with lots customisation. Cons, the realistic art style, bugs!
TS4 is like buying my first house, where I need a lot of money to buy/build my ideal house :D. The pros, the cartoonish art style, for some packs, it's kinda detailed (parenthood, vampires), easy to build houses (TS4 is the first iteration where I built my own houses), lots of free updates, interesting new gameplays. Cons, most of the pack has lacked of details, no cars, I feel disconnected with my Sims (no memory system), lots of oversized stuff (I just want a normal telescope & microscope: s), lack of customisation (I want to build my own world too, my own concept, my own fantasy...) ;)
Overall, I really love this game.