Ah the purchase units phase, so many possible units to buy, but are you buying the right ones? Believe it or not, you may be buying the suboptimal choice.
When it comes to land warfare, pairing Infantry and Artillery are the best way to attack and defend. As Corporal Clegg's video "Artillery vs Tanks" shows that when paring an infantry and an artillery, which increases an Infantry's attack to 2, the same as the artillery. Together, these two units have a 55% chance of getting a hit at two or less, as opposed to a 50/50 from the tank at three or less. According to the Axis & Allies calculator by David Skelly, in all three versions tested (Anniversary, 1940 & 1942) with the Infantry and Artillery combo, against a single tank, these two units have a nearly 80% chance of victory. But that's only single combat with a 2 against one battle, we need more money for this to be a fair fight. In Corporal Cleggs Video, he gives 50 IPC's to spend for 3 different army builds. Of these builds, the best performer is the Artillery and Infantry combo with every infantry paired with an artillery. In all versions this army build has between a 55 - 60% chance of winning against a opposing force made of a mix of units. Monetarily, the Infantry and Artillery combo costs 7 IPC's, with a tank in most versions being 6 IPC's. (except in anniversary, where it is a dollar cheaper) To show that this strategy works when buying in bulk, (in Global 40 at least), Reddit user u/TheeBiscuitMan was, at the time when they wrote the post about 4 years ago, 29 wins - 0 losses with an artillery focused strategy with a G3 Barbarossa with a Southern Push. Now that's impressive.
When it comes to Naval Warfare, Submarines, Destroyers or a combination of the two for a fleet is the best. As shown in the Corporal Clegg Videos "Best Sea Unit" he pits around 100 IPC's worth of fleets against each other with the fleet of Submarines and Destroyers coming out of top, followed by a balanced fleet. In terms of monetary value, submarines are your best bet, costing only 6 IPC's and having the surprise strike ability and being able to go where ever they lease unless there is a destroyer in the way. This leads me to believe and argue that destroyers, despite costing more, are better since they negate a submarines surprise strike, can shoot at aircraft, and block a sea zone, when combined I think these benefits for a destroyer out way the benefits for a submarine.
Now, I've given plenty of evidence why you should mostly be buying infantry, artillery, destroyers and submarines, but i'm sure there are a few of you thinking the opposite of my view, that's good, civil discussion is a lovely thing. I believe some examples of opposing views with basic arguments goes like this: "But tanks can blitz, and have better range! Cruisers can bombard, battleships take 2 hits and carriers are useful offense! And what about planes?" Obviously every unit has it uses, I'm not saying you shouldn't buy these units, I'm just saying you should buy certain units more often. As I saw on a forum post about the Soviet purchasing strategy, most players (at least those who were answering in the forum post) were buying infantry, artillery or a mix of units. I'm glad everyone has their own reasoning and play style that we can discuss. Discussions like this help new players to get the hang of these complex games and can also help advise older players on how to hep those new players.
In conclusion, I want to quote Don's Essays, now I must note that I got this quote I got from another Corporal Clegg video since the link to the Don's Essays site is a dead one. Anyway, the quote is as follows (Quote is at 3:09) "Any player who does this strategy will always win, regardless of strategy, and any player who always wastes their expensive pieces on the front lines will always lose, regardless of strategy." This quote is referring to mostly buying infantry and artillery vs tanks and is a perfect sum up of my argument and beliefs. If you play smart and only try to lose your cheap, expendable units, like infantry and artillery, you'll cruise to victory in almost every game you play. Of course, you can play however you want, I'm not forcing you to change your whole play style. I'm just giving you hopefully helpful information regarding how to win more in Axis & Allies, it doesn't matter if your new, old, or somewhere in the middle, I hope this site is of use to you. I would lastly like to say, please use the Axis & Allies battle calculator by David Skelly when playing, it's a great tool.