"Best" is entirely subjective. "Biggest" would still be QTP3. "Most efficient" would be something like Vibrant Textures or Mtgr Textures, with or without the addition of Bomret's Detailed Normals for Vanilla, despite the name. Because, in those [four] cases, there's absolutely no increase in demands on hardware, but aesthetically, there's quite the "upgrade" (if you happen to like any one or combo there).
Personally, what I recommend doing is mix 'n matching. Well, that's what I do, and I'm pretty happy with the results.
Whatever you choose for textures (and maybe normals, and maybe also meshes) you might want to check out OBGE v2. It's kind of a "plugin" really, that requires OBSE (Oblivion Script Extender). With it, you can apply a whole variety of shaders to whatever Oblivion (plus your graphics card) is spitting out to your monitor. Just before it gets there, more or less. Things like Ambient Occlusion (for shadows), Anti-Aliasing (for getting rid of "jaggies"), Depth of Field, Colour Effects (e.g., Saturation, Gamma, Contrast, etc.), and so on. Well worth a look, at the very least, because even with vanilla Oblivion, it can make everything look *that* much better. I actually tried just that the other day, and yes, it makes the
world of difference.
The other thing to watch out for is DistantLOD textures and normal maps.
Essentially, have a look http://devnull.sweetdanger.net/obliviontextureoverhaul.html (rather out of date, but still very useful), and http://tesivpositive.animolious.com/ (more up to date, and also very useful, and not just for graphics-type stuff).
Also, it's worth a bit of time to search through TES Nexus as well. For example, go to "Browse categories", then "Models And Textures", and sort as you like (e.g., File size, Downloads or Endorsemants, descending). Scan some pages, see what comes up. Maybe even have a look at the most recent uploads (again, via custom sorting) or most recently updated files. Try some searches by key word - and note, partial words
do work.
Let me/others know if there's anything else you want to know, in particular.
Best of luck.