No offense, Rubik's Company, but absolutely no cube that says "Rubik's" on it belongs within 25 feet of a speedsolving competition. These are more for collection/for-fun/modding/etc.
That out of the way, we can begin! I stand by VirusBoi's suggestions, and I add this:
V-Cube 2 is underappreciated from what I've seen. While I too prefer the WitEden Type C-II, I haven't been disappointed in this cube.
For the 3x3x3, I found MF8 Legend and Type F-II to be phenomenal puzzles for their cost. I still use a Lubix Fusion (mod based on the Gu Hong). The Lubix Elite is one of the most phenomenal puzzles I've ever played with (but I don't own one
V-Cube 4, Dayan+MF8 Mini, Meffert's. If you really want to get picky, this cube has some of the biggest KO variety, but believe me, they are not worth the trouble experimenting with.
V-Cube 5. Use tutorials online for modifying this. At first it seems kind meh, but with some work, this becomes one of the strongest in the set.
The 6x6x6 is annoying. Get whatever you wind up getting (which should be the V-Cube 6, as you won't find many legal 6x6x6 cubes)- no matter what, it's going to end up being a huge disappointment.
The V-Cube 7 is my favorite puzzle. No one's ever going to convince me otherwise, and people have tried and failed to impress me with anything other than V-Cube.
Nothing mass produced above 7x7x7 is legal, but then again, nothing above 7x7x7 is WCA-standard anyway.
Note: The V-Cube 3 is the only one that I don't recommend for use competitively. It is great, but it's nothing special in quality. For sure, if it's either this or Rubik's, get this. But, pretty much any cube that costs more than $1 and doesn't say "Rubik's" on it will one-up that puzzle. Might as well get it to complete your collection, though.
Andreas Nortmann wrote:
Things like this are illegal.
If not I will pass an appropriate law.