you can't get any more original then Adaptoid. But I would go for a nice xbox 360 controller as they fit great in hand and the usb adapter is easier to find.
wow, I didn't know retrolink made a n64 pad.
I'm not sure but it's a heck of a lot cheaper than what an Adaptoid would cost if you were able to find one.
The Retrolink pad would probably be a good trade-off as it is a close clone of the original and cheap to buy. It'll work with emulators well enough as long as you have proper drivers for it.
Adaptoids are really expensive now on ebay, but you can use your original controller and I believe they support N64 controller accessories as well (at least in theory).
I recently purchased a lot of 12 of these adapters to rip the microcontroller chips for a project I'm working on. I've found that most of them have issues with one of the sockets (rapid-firing buttons, dead axis, or button crossover), but all of the ones which had problems with one socket, the other socket worked fine (at least for the 12 I have). Just something to keep in mind if anyone is thinking of buying one of these for multi-player since they have two sockets. You may end up with one socket being unusable.
To my knowledge, most game controllers and adapters for PC nowadays are generic USB HID devices, which should work fine in most modern Linux distros. I don't know if the Adaptoid was one such device since I don't have one. The adapter I mentioned earlier in the thread is and is well rated, but note paulscode's comment as well.