I voted for LOVE PUNCH because it's my favorite.
It's the best choice if you love poppy music over a "cutesy rock" style. It also has her best selling single ever, "sakuranbo", which is one of the cutest/most annoying songs in the world.
The ballads probably aren't as good compared to LOVE COOK or LOVE JAM, but they're not all dark/sad like in those albums.
I think it also has better vocals than LOVE COOK or LOVE PiECE, because although she was squeaky at times, she just seemed to control her voice better.
The booklet is also very cute, since it's filled with jelly beans and milk and lollipops and random vegetables being used as balls for sport things.
And in my memory, there was at least one picture on every page...
After LOVE PUNCH, I would say LOVE JAM. She's evolving more towards the "cutesy rock" style there, but it also has "daisuki dayo." and "kingyo hanabi" which are pretty nice ballads.
LOVE COOK would be my third choice. It has some nice songs, but I find some of them so annoying.
But everyone seems to like the singles from this album, so.
Like: neko ni fuusen, Planetarium, Cherish (which isn't a single, but everyone loves this song apparently)
Lastly, LOVE PiECE. It has some good songs like mirai taxi, yumekui (which probably has her best vocals to date). But to me a lot of the songs on this CD sound really alike to each other or like songs from LOVE COOK.