Here's one I haven't thought about in a long time but I think I really enjoyed when I watched it: .hack//Sign. I guess it's been nearly 15 years since I watched it; hardly remember a thing about it in terms of specific characters, plot points, etc. But I recall liking it. I don't think I went on to watch anything else in the .hack franchise though. This was back in the bad old days when I didn't always know where to look for anime online, still had spotty Internet access, etc. I'm not even sure how I lucked into finding the whole series available for download.