Quite simply, I take two or more games that I think are more or less equal and compare them. But instead of the overall picture, I compare the points that are close enough to properly compare.
Some of the points are my personal opinion, while some are points that I've noted other gamers comment on.
Also, if I don't comment on comparable points, that means I think they are equal. And that means the majority of points for these games, which is why I consider these games mostly equal. Even if a game has more superior points, it's just a fraction of the overall game. So this should NOT be taken as a way to claim one game is overall better.
This time I'm comparing the Castlevania games on the Gameboy Advance. This is because not only are they in the style of Symphony of the Night, but I've played all of them, whereas I've only placed Dawn of Sorrow on the DS.
Anyway, this is where each of the three games bests the others:
Circle of the Moon:
1. The music. Just comparing the title screen music of the games should make this clear. Harmony has a bunch of chimes that seem like out of the sound chip of the first Gameboy. Aria is better, but not that memorable. Circle has a synthetic chior. It's not perfect, and you can hear some minor crackling, but otherwise it's great. The rest of the music is just as wonderful.
2. Best spell effects. While the card system overall left a lot to be desired, especially with how they were distributed (good luck finding most without a guide, and sometimes even that's not enough), the effects when you had enough cards were just awesome.
3. Best extra game mode. While the multiple modes after beating the game are just variations of the main game, Maxim in Harmony wasn't as balanced as Julius in Aria. But what gives this the edge is Magician Mode. You get all the cards, and can now try all the awesome spell combinations. Many are enough to overcome the reduced states (like elemental attack absorbing).
Harmony of Dissonance:
1. Best dash system. Cirlce had double tapping, which works, just not as well as in this game. Aria had the back dash, but no foreward dashing. This just has you press the shoulder button in the direction you want to speed towards.
2. Best challenge balance. Cirlce was often challenging for the wrong reasons. Aria had some good challenge, but still got really easy once you found the right items (Claim Solas especially). Harmony is challenging at just the right parts, and by the time you get the most powerful spells, the bosses have grown more powerful in kind.
3. Best second form for Death. Circle had a skeletal snail, which was a good challenge, but just not as cool as it could have been. It just didn't seem to fit for Death. That should have been its own boss. Aria had the second form turning into... Death, where the first form was his scythe. That's just not right. This game had him turn into a flying, snake-like skeleton, whith scythe arms. Now that's how you do this.
Aria of Sorrow:
1. Best overall graphics. The graphics on the other games are already pretty great. Just Circle loses some points for being too hard to see without a system light. Harmony worked just as well as Aria's, but just didn't have quit the complexity that put this at the top.
2. Best magic system balance. While the souls rarely have as awesome effects as the cards in Circle, they are easier to get, and offer plenty of room for experimentation.
3. Best story. Even better than Dawn of Sorry, but this is among the other two. While all had good stories, this wins because of the twist. Some of you might not know this yet, but it's a doozy. A freakin' great one.