IMO, octavers are such a personal thing like OD's or fuzzes. There are a number of them and you never really know which one youll like the most till you try it. In saying that, here are a few I can think to try off the top of my head:
Boss OC2- a classic used by many, hated by a few. Its a very, IMO, organic and glitchy type of octave. Not the greatest tracking one around by any stretch. The OC3 is pretty cool too, it has the Poly option and the OC2 option (which doesnt capture the old OC2 that greatly IMO).
EHX POG2/Micro POG- What a fun octave!!! Ive owned a mini and have the 2. They track almost flawlessly and are really handy in so many ways. On my POG2 I can get a really deep n nasty sub octave going that will shake the room to its foundations

It can make your bass sound like a 12 string guitar (sort of), a sub bass or an organ. Or you can get a simple octave going. It is pretty digital sounding though. It is a digital poly octave afterall
MXR Octave- I really liked mine when I had it. Organic, tracks better than the Boss, sounds really good and has that little button on it which helps you cut a little better. Very handy and musical sounding IMO.
Roctron Octron- I have never played or had one but I hear it simply rocks and I thought I should mention it.
EBS Octabass- This is a nice ocatve too. Its clean sounding but a little sterile to my ears. Havent used it much though.
Aguilar Octamizer- Ive heard nothing but a lot of good about this one too and always wanted to try one but still havent. Thought I should mention it too.
And of course the DigiTech BassWhammy- Talk about fun! Ive actually thought of ditching my OC2 and POG2 and just use the BWhammy. It can do a lot of octaves and other stuff. Its tracks really well and even can be used as a chorus. Win win IMO.
I have and use an OC2, BWhammy and the POG2. Im very happy with these. With them alone I can cover almost any octave you could think of. There are some really good ones out there too that are not in the "mainstream" so to speak so really shop around if youre interested and want to find "the one" for you. Its out there. And they are def not all the same. Try as many as you can!