Is the alekhine's defence advantageous or disadvantageous for white?
Why is e4 so commonly played in the first move?
Advantageous. You get to kick around that black horse several times, gaining tempi. But don't get carried away as all the pawns that go forwards will become targets eventually.

Yeah, if you see, the Sicilian is interesting but difficult according to me, and I also find the pirc defense and reti interesting.
The Pirc is a messed up Philidor's Defense. Play e5, and then d6, and then Nf6. Some guy wanted to become famous by messing up the order of a decent counter to e4.

Reti is interesting, but way too complicated. There are so many more logical responses, because it is far more passive than 1. e4, c4, or d4. 1. f4 is an insult to chess.

1. f4 is an insult to chess.
1.f4 is a good move, under the concept of playing a reversed tempo-up Leningrad Dutch. Equal? Sure it is, but with proper play all sound openings result to equal positions- and the Leningrad Dutch setup is complex and unbalanced.
Yeah, if you see, the Sicilian is interesting but difficult according to me, and I also find the pirc defense and reti interesting.