1.c4 e6 2.Nc3 d5 3.d4 (2...Nf6 3.e4) can work if you're happy to transpose back into 1.d4, but want to avoid the Nimzo and any kind of Benoni. Likewise, 1.c4 Nf6 2.Nc3 g6 3.e4 and a Grunfeld will not appear on the board

You pretty much answered your own question - the Reti is easier, but as with any move order, there is a price (1.Nf3 d5 2.c4 d4). As noted, the KIA is easier again as there aren't many setups where it will be a bad choice, but CarriedbyGg is right that it will be a much more effective tool if you learn to play all the different structures (d3/e4, d4/c4, d4/c3, c4, b3/Bb2) rather than sticking to one setup against everything, which could be limiting for your game in the long term.

The London might be the easiest opening of all to play, but that's another section!

CanadianClub wrote on 07/21/17 at 22:36:57:

I guess this is a problem if the Reti is your main repertoire and you're aiming to press for a theoretical edge. But I use the Reti as many do, as a break from heavy theory or when I think it will be especially uncomfortable and/or surprising for my opponent.

However you play them, the positions after 1.Nf3 d5 2.c4 d4 are unbalanced, not prone to early simplification and still not that theoretical - what's not to like?  But then I've been interested in Benko and Benoni structures for a long time; when I see them often my first thought is I would be happy playing either side (unless one side is already clearly better, of course).

Btw. Your description of the English/Reti sounds like something I should play - I usually enjoy and do well With dynamic pawn structures. But when I tried 1.c4 some years ago, I just got draw upon draw out of it... probably gave it up too soon.

Eric's whole answer is good but the line "The key is to balance the amount of theory with what you can handle and what your opponents are going to throw at you" really summed it up. How many of your opponents really do out-theory you ? Stick to a repertoire ad you'll soon become the knowledgeable one. I happen to prefer the English but there is nothing wrong with 1.Nf3. I would recommend a repertoire book and then to change individual lines if you're not happy with them. That's a better approach I would say than trying to play a "system". Mostly the people that write the repertoire books are very strong compared to us amateurs. Why not use their opinions and then supplement them with experience ?

As an amateur player looking for the easiest way out of the opening, I found that the Reti might actually be easier than the English. I like a one-size fits-all approach to the opening and don't like to memorize lines at all. So I would never play something like the Nimzo-Indian because White can avoid it by playing 3.Nf3 instead of 3.Nc3. I would never play the Czech-Benoni, despite how great it is to lock the position and not have to memorize anything, because White could simply refuse to play d5. So I always looked for simplicity in the opening and a way to avoid main lines as much as possible. This meant that when I played the English I would not transpose into things like the King's Indian, Catalan or Slav under any circumstances. This was harder than I thought.

1.c4 e5 2.Nc3 Bb4 and 1.c4 e5 2.Nc3 Nf6 3.Nf3 Nc6 4.g3 are just too ambitious for me. On top of simply not capturing on c3 at all, Black has all kinds of ways to play these positions leading to pawn sacs and really complicated positions that I didn't want to go into. On top of this, Sicilian players would have their experience to help them here.

1.c4 e5 2.g3 Nf6 3.Nc3 isn't much better. After 3...c6 4.d4 (otherwise Black either owns the center or gambits a pawn leading to a really dreary game) ed 5.Qxd5 d5 6.Nf3 Be6 I don't feel comfortable in the IQP position or the gambit lines either because I wanted to avoid complications in the opening.

1.c4 Nf6 forced 2.Nc3 because 2.Nf3 would lock me out of the Botvinnik system and other replies to non-...e5 moves. And after 2...e6 I found that I was playing one thing and against 1.c4 e6! I was playing something completely different! This is obviously not acceptable to me. I just played into the French exchange with 4.c4 here which was something.

...c6 is no different. There are so-called anti-Slav systems but setups with ...g6 practically force White back into main lines which again was not what I wanted. I think I just went into the Panov-Botvinnik against ...c6 because there were at least some sidelines that were ok to play.

And then there's 1...c5, 1...f5, 1...b6 and wow it was just too much.

But 1.Nf3 is so much easier!!

1.Nf3 d5 2.c4 e6/c6 3.b3 and I'm done! Ok I should look at games but that's all I have to do!

1.Nf3 d5 2.c4 dc 3.Na3/3.e3 and oh my I'm done again!! Anyone can play this stuff!

1.Nf3 c5 and ok so there's no free lunch here... I just play d4 as early as possible, put pieces in the center and hope nothing goes wrong.

1.Nf3 d5 2.c4 d4 and there are some problems... I don't like the Benoni stuff too much at all which is odd seeing as White is practically inviting it. 3.b4 is also kind of crazy too so I just bite the bullet here and play 3.e3 Nc6 4.b4. I have no idea if this is good or not. All I know is that White's play is natural regardless of what happens and that's fine with me. 4...de 5.fe Nxb4 6.d4 e5 7.Nxe5 Qh4+ 8.Kd2 is admittedly crazy. But I think it's fine. You get similar positions in the king's gambit after 1.e4 e5 2.f4 ef 3.Nc3. I think it's ok as long as I move the king at the right time.

1.Nf3 f5 2.d3 and play like Carlsen. Easy!!  If Black delays ...f5 then simply play for d3 and e4 anyway.

1.Nf3 b6 2.g3 is just going to transpose into 1.Nf3 e6.

1.Nf3 g6... king's indian... Here I just play the reversed closed sicilian. It's the only thing I'm really uncomfortable with because I don't understand sicilian positions well.

But that's it really. But I've seen so many people talk about how easy the English is and how you just play Rb1 and b4 against everything but it seems a lot more complicated than that.