I play on Neopets and Marapets, as well as Powerpets, Shining Stars, Club Penguin, and various other virtual pet sites.
I like Neopets best, partly because that's the first one I found, but also because it seems more interesting and fun that Marapets, and I have more friends on there than I do on Powerpets.
Marapets feels like it looked at Neopets and decided "Tombolas are popular there? So we'll have more Tombolas! Where Neopets has 1 tombola, we'll put 4 in, and that will make us more popular than Neopets because Tombolas are popular!" or "Faeries are popular? Hmm...how many faeries can we put in our lands? Well, we need at least one for every land, of course. AT LEAST." and "Huh, they have what, 20 lands? We can do a lot more than that."
It feels like Marapets deliberately tried to look at everything Neo did, and take it to the most extreme. But the term moderation is useful here. Not to mention, I really dislike that it costs real money to get any of the limited edition pets. My mom and I each have a lutari. She got one because she chose to get a game on her cell phone, however, I got mine by having the Pet Creation window open for three hours on Lutari Day. These are both valid ways of getting a lutari, and while the second is more difficult, it is possible and no one need spend money. On Marapets, however, a Chibs, for example, being one of the most highly valued pets, can only be received from an account upgrade.
Powerpets doesn't have most of these things...their games, while not completely new and original, aren't incredibly obvious transfers from one site to the other (Meerca Chase became Murfin Chase...they could have called it Murfin Run, or Murfin Catch, or something, rather than changing the pet name and leaving it) and they are reasonably fun. The points aren't hugely difficult to earn, but they are more helpful for that. It seems like on Marapets, items are priced very high due to how easy it is to get points. I have five million mp, and can't buy a chibs potion still. Powerpets at least has a good message behind it, and donates to shelters and other funds for real animals. The creators are fairly kind, and they definitely play on the site-one of the two creators of the site bought an item from my younger brother's shop just a couple of weeks ago.
Shining Stars is a kid's site. It's fun! I don't go on much, I have three plushies on there, and it's pretty fun just to relax.
And Club Penguin is pretty cute. I don't have a membership, so I don't go on much, but I donate every coin I get to the donation stations in December. If nothing else, I like that they donate to help people and that they let the players choose.