I guess it depends on whether you want information, or if you want a picture guide with a checklist, or what you're looking for.
MLP Wiki is the one that covers most generations (it is not good for G4). With such a huge project it is still a work in progress and has gaps and errors, but is a great simple resource to use if you just want a name or at a push, an accessory.
Speaking of accessories, though, if you want information on those, the better accessory resource is Chrissytree's.
https://mylittleponyaccessories.weebly.com/The Arena here has a gallery for what are known as Nirvana ponies - mostly ponies made in other places than Hong Kong/China.
http://mlparena.com/archive/gallery2/main.phpThis is not a checklist per se but has a lot of images about different versions of ponies in G1.
I also have a website, though mine is written from a UK perspective and it doesn't have a checklist, so may not be of use to you overall. But it is more accurate than the Wiki in handling UK and European releases.
http://www.etherella.com/scrapbook/MLP Merch is great for G4, I would avoid it like the plague for G1 as although it has improved, it has some very wrong information still. That includes the checklist.
Ditto Strawberry Reef which is great for G3 stuff but again, not great for G1 (and no longer being updated).
Lavender Lagoon is awesome for G2.
Basically the message is, use as many sites as you can if you want accurate information, and don't try and do everything on one site.
And at the end of the day, if you have a question, you can always ask people here
Someone will know the answer.