I knew pretty much right away that I would love Buffy more than anything. I often times find myself wondering the same question that you are, I just NEED something to that level. I haven't found anything to 100% mean what Buffy meant to me, I don't think I ever will, and that is ok to me. But I have found some things that have 95% meant what Buffy means to me and I will list them here in hopes that someone out there may find these things as meaningful as I did...
TV Shows
Angel- Almost didn't mention this for obvious reasons, but seriously, if you are new to the Buffy world or if you've just never seen Angel, go do so!
Alias- At first I didn't think that this show was going to do it for me, but about half way through the first episode I changed my mind. I thought it would be good, but not nearly as good as Buffy or in the same way, but I was wrong. As the seasons progressed, it became an AMAZING show that included action, drama, romance and a little bit of fantasy. I truly love Alias and recommend it to anyone looking to recapture a tiny bit of the Buffy feel
Firefly- Well duh... if you are on this forum... you should know... and if you don't... you've read enough of us saying it's awesome that you should just go now and watch it!
Point Pleasant- Another short lived show. I really liked this one, it was Marti Noxin's original show, but got cut from... guess which chanel!?! after only 5 or 6 episodes. The DVD set includes all of the episodes, 12 or 13 total I think. But even though it's good, it is about as depressing to start as Firefly because you KNOW it's going to end soon
Books
Twilight- I know a lot of you don't like this book, and that is perfectly ok with me! But I really really did, it touched me in a way that no book EVER has. If I had to pick one thing that meant as much to me as Buffy, it would have to be Twilight. I originally didn't want to read it, I had heard a lot of complaints about it and was automatically biased against any kind of vampire fiction, after Buffy who needs more? But I was wrong, and I am happily one of those geeks who has read the books over and over and over and over! If you haven't read them you should at least give the first book a try, then you'll know if you hate it or love it!
Harry Potter- My original Buffy, I found Harry first. Harry's world is a magical place. The first book, and maybe the second are a bit kiddish, but worth the read. As the story progresses, the books get more "grown up" and believe me they are WELL worth getting to. Before Whedonesque.com I was a member of Mugglenet.com checking in daily for my JK Rowling fix. If you liked Buffy, and you enjoy a good fantasy book, give Harry a try!
Video Games
Final Fantasy X- I know that this sounds just a little strange, but there has been a video game that has meant as much to me as Buffy. It's Final Fantasy X and it has a sequel which isn't quite as good but just as fun to play Final Fantasy X2. It's an RPG that takes place in a world unlike anything you've ever seen! It's got love, magic, epic battles, good vs. evil and so much more. If you have a PS2, I think this game is $10 or $20 because it's a "greatest hits" and I would really give it a try!
That's all I can really think of for now. Again, this stuff may not be everyone's cup of tea, but all of the things I have listed have enriched my life and might do the same for you.