Hmm, I've watched more anime than manga, but I'd say Miracle Girls and Yotsuba!. These were the first ever series I've finished. Miracle Girls is very straightforward, but very well written and has inspired me to story write and draw to this day. Yotsuba well, is just adorable always <3
This was hard to think of cause I watched SO MANY cartoons as a kid and loved so many, I'm willing to rewatch em all,
But I'm going to go for Pingu~OR Kipper Can't be to sure. They were both wonderful.