I would argue that 4p is the "best competitive way that still keeps the spirit of the game" while 2p is simply the "most competitive", but I wouldn't mind having one season as 2p.
6p and 8p are out of the question, besides the long waiting times, on high player counts getting duplicates is harder and no deck is super strong, so luck takes even a bigger factor. 6p and 8p are good for party games but not recommended for Arena.
Comparing 2p and 4p.
Better 2p
- More fair drawings, you always play the same opponent while in 4p you may get into the final by playing the weaker player a third time on R7.
- Strategize against your opponent's deck, since its always the same deck you can counter draft.
- Fans cards are not useless (you don't need to win the final).
Better 4p - As said in general 4p keeps the spirit of the game
- Your deck has to be robust to beat different decks.
- Many players are drafting, so you have to keep an eye on what strategies might be free.
- There is a real tournament going on, with a final game and no chance of an early win.