The Ultra 64 is my "home console". By that, I don't mean it's the one I have at home (though that is true), I mean it's the console I grew up with and essentially consider my home turf—see Caddicarus and the PS1 for an example of what I mean. It's my favorite console of all time for two factors, one it's an ingenious design and two I grew up with it so naturally I'm biased toward it. To this day it still has the graphics that most aesthetically please me of any console. Until the Ultra 64, consoles with joysticks often only had them because they hadn't thought up the D-pad yet. The Ultra 64 had one because it was trying out 3D gaming and as Star Fox proved on the SNES before it, a D-pad isn't enough for 3D gaming.
It's safe to say that the Ultra 64 is one of the most important consoles in gaming. Not only did it introduce the concept of the 360 degree joystick, but even if it didn't introduce the world to 3D gaming (that dates back to the 1980s), it certainly raised the bar for both gameplay (thanks to the joystick) and graphics (thanks to the RSP and RDP, games on it aren't aliased, wobbly, and choppy). While it was nowhere near the top in sales figures, it's king in the long run, and is today the console that holds up to scrutiny the best.
I'm definitely not biased at all here.
The 64DD (often incorrectly labelled the 64 Disk Drive) was an addon that had the potential to completely change the U64 as we knew it. Magnetic disks are generally seen nowadays as an ancient relic (alongside cartridges... funny how that plays out), but they would have been a fantastic addition enabling larger and more fleshed out experiences for much cheaper than a 64MB or even, dear god, a 72MB cartridge might offer.