"IG and SW are the best armies!"
No, they aren't. Seriously. If you follow the guidelines form the 40K rulebook, they aren't any better than any other army.
"But IG and SW win a lot of tournaments!"
Yup, they do. A disproportionate number, I would guess. Why do you think that is?
I don't know for sure, but I have a guess: terrain and setup.
What does that mean? It's very simple; different kinds of setups and different amounts (and types) of terrain will affect armies in different ways. That seems really obvious to say out loud, but judging from many of the tournaments I've been to and having talked with different TOs, I don't think the connection has been made for a lot of them.
Of the three basic deployment types, with each there is a different balance for armies. Pitched Battle is the "default" setup, with a 24" minimum separation and 36" maximum separation. (I'm assuming straight across, as opposed to a refused flank, and also assuming at least one party has an interest in closing the distance.) Spearhead, with its much larger maximum separation, favors shooting armies- they can back into a corner away from the enemy and get an extra turn or two to pound on them, which can be critical. Even holding in reserve is no real protection against this, although outflankers can help somewhat. Dawn of War, on the other hand, gives a major advantage to melee armies, as it makes it possible/likely to start within 18" and effectively turns off shooting during T1. (Walking in from a back edge is painful, but melee armies should have ways to close distance more quickly or they're trash anyways.) With the three setups overall being even between different army types, no one army type is particularly favored.
Terrain can likewise drastically shift the course of a game- my TSHFT report would be one example (to whine just a little bit more), but the relative amounts (more/less than the recommended 25% coverage) and types (area terrain, ruins, forests, dangerous, hills, impassible, etc) can make games very different. Of course, this is, in most ways, a good thing, as playing the same game time and time again would be very boring, but some terrain types also favor certain armies. Tyranids and Orks like tables with lots of area terrain coverage to make up for weak saves; Eldar and DE prefer chunks of blocking terrain that divide the battlefield off, allowing them to destroy the enemy by pieces while using their speed to avoid engaging the rest. IG and Tau like tall pieces that give them excellent vantage points onto the battlefield. And again, percentages will favor different strategies; crowded tables are a haven for assault armies, who can make best use of the cover while closing. Emptier tables give shooting armies a leg up, as there is less to get in the way of LOS of the low-AP guns that tend to be most effective weapons.
How do each of these things turn tournaments towards IG and SW, then? IG are always a shooting army, obviously; SW are as often as not, and rarely do they leave home without at least 3x5 Long Fangs.
Deployment is one that tends to get screwed up without anyone realizing. Many tournaments use alternate, nonstandard deployment types to try and mix things up a little; there's nothing wrong with this- in fact, I applaud it, as the basic ones can get boring after a time- but often they are not well thought out. TSHFT, for example, uses two extra deployments (for a total of five): short table edges, with the board divided into thirds (two deployment zones and a no man's land) and diagonals (measured 18" from the "enemy" short and long board edges). It shouldn't be hard to figure that the short edges deployment favors a shooting army, giving it pretty much a full 48" of open space even under the worst of circumstances (as well as negating the strategic concept of refused flank, giving fewer options). Diagonals, as implemented there, also tend to favor shooting armies- it's not hard to make 36" or more of table between your opponent's forces and your own, even when considering the possibility of units entering from reserve on the long edge. Though most diagonal setups are slightly less punishing, they still give more than Pitched Battle's 24-36" of space, giving shooting a distinct advantage. There are some other setups, but those two variants are by far the most common.
Terrain, even more so than deployment, is where the real issue comes in- it magnifies the effective distances melee armies have to cross, as open spaces on the battlefield are places where they tend to get cut down due to lack of cover. Now, it's somewhat understandable, as terrain costs money and takes work, etc, but honestly, if you're going to complain about overpowered/cheesy armies then you really need to stop handing them advantages. Not all terrain has to be spectacular-looking; there are blueprints for cardboard buildings around. You can make a dozen hills or forests in the course of a couple hours. Aquarium plants glued to a base may not win any awards, but they aren't going to break the bank or suck up your free time, either. Hell, there are a lot of things that a little bit of gluing-together and spray paint will make into entirely serviceable terrain. Many players also build terrain- or perhaps might like to; why not offer some amount of store credit for terrain pieces donated? (Some players will donate regardless, but it's nice to have an incentive; more flies with honey and all that.) It really isn't that hard to make a terrain collection if you set yourself to it, and doing so makes games more interesting than simply "stand still, shoot my big guns, pull guys off the table." 5th edition was all about moving away from that, about making the game dynamic and interesting again.
