Fun series, containing 3 decent games and one amazing one.
Tokimemo 1 is a comfy, fairly realistic game in which you enjoy innocent romance with a variety of cute anime girls while trying to keep your grades up and get through Japanese high school without dying of stress.
Tokimemo 2 is hands-down the best in the series. Gameplay-wise it's very much like the first one, but instead of being in a sweet slice-of-life anime, you're in a crazy comedy. Your girlfriend might turn out to have an evil twin, or be hit by a truck on the way to school (multiple times) - but she's fine. The school play during the culture festival one year is a parody of Lone Wolf and Cub. One of your potential girlfriends is an obnoxious rich girl who brings her butler along. There are a LOT of different girls to choose from, and all of them are cute in their own way.
Tokimemo 3 was kind of a step down; they switched to 3D graphics instead of 2D drawings, and as a side effect the number of girls dropped enormously. The previous game had over a dozen girls, while 3 has 6. Also, they added this horrible system where you had to choose what to wear on dates, which greatly effected the girl's impression of you: all the clothing items names are written in kanji, so unless your Japanese is a lot more advanced you'll have no idea how you're dressed or whether it's "right" until you arrive on your date and she makes a face at you. I've still managed to finish the game multiple times with a lot of saving and reloading; the girls are still cute even if they're 3D, and some of the story elements are fun. (The apparent "delinquent" girl has a plot twist so crazy it feels like it belongs in 2).
Tokimemo 4, so far the final entry, fixes everything wrong with 3 by bringing in features from the parallel Girls' Side series, turns the graphics back to 2D (though they're kind of Live2D animated), and gives you a much better idea of what you're doing. Unfortunately, it also goes back to the storytelling style of the first one (a lot more realistic and less crazy/funny). And worse, the art style is "modern". Gone are the adorable showa-era designs of the previous games; everyone has Kyoani looking faces and bland, realistic hair colours. Still fun. Haru is best girl.
There's also a parallel series, "Tokimeki Memorial Girls' Side", that's the same kind of game but intended for girls, where you have a variety of boys to try and win over. They're, again, more like the original game, less wacky. Cute art style though, especially the weird chibis that look like something a Japanese schoolgirl might doodle in her notebook.