This is a difficult question to someone like me as I haven't found that ONE favorite game, but rather have numerous games that I would consider a favorite. I would only consider a game a favorite of mine if I enjoyed playing it start to finish, one that I actually aim to beat completely and I would have liked to play it all-over again. I'll just list some games that I consider a favorite recently.
1.
Wario Land 4: I think it's worth noting that, despite what I have said above, more and more I realize that this could be that one game. Wario Land is a cohesive series with solid games, and Wario Land 4 is no exception. In fact, I could argue it's the best in the series. Wario Land 4 isn't your average platformer, as there are no pitfalls or even that many enemies, and the only way to die is when you lose your life (which is pretty rare actually unless it's super hard) and when you don't make it back in time. No, Wario Land 4 is a platformer mixed with puzzles and a sense of exploration, where the main goal is to collect treasure in a large level and then making it back to where you started after activating a bomb at the end which is necessary. The amount of secrets and stuff to do in one level alone is huge, and takes about 15 minutes MINIMUM if you do try to collect everything you see.
The level design is genius. There are only 18 levels in the game, but they're all exceptionally different and huge that it doesn't matter, and even when you play them at a higher difficulty the items are actually in a different place, so it's like playing a whole new level. The visuals of these levels are also expertly done, like how one of the level is like playing inside a volcano and the background consist of lavas slowly flowing on rocks, and there are also effects that makes the level seems like it's boiling. But once you activate the bomb, the level turns from a volcano to a snow mountain. All the lava is now frozen and even the enemies changed. It's things like this that makes you appreciate the design decision in the game. Another thing about the level design is that the enemies aren't just randomly placed and that they are there for a purpose, be it for annoying you, damaging you or the best part...helping you. The level design is so good that it tells you to use your enemies to your advantage, be it by making them power you up (a lot of them do that) or just using them as an object.
I can go on and on about the level design but it really isn't the only thing this game has. The game is full of charm, and it really screams Wario, especially the music. Like when you activate the bomb, Wario screams to hurry up and the timer start where
the music starts slow and gloomy adding to the tension, but then gets faster and a lot tense.. Also not only is the music good, but it fits with the atmosphere and level so well, which a lot of platformers lack. (Like Donkey Kong Country for example. Amazing music, but not my choice for a game starring DK, so I actually hate it.) The Arabian Night level for example, where the level itself introduces to a gloomy arabian village at night with big buildings, and the
music just fits so much with the atmosphere.
The game isn't linear either, far from that. After you complete the first chamber, you unlock 4 other chambers, each with a set theme and 4 levels and a boss. But just because each chamber has a specific theme for it, doesn't take away from the variety of the level. For example, the Sapphire chamber, which is set on the horror theme, has a ghostly village level, an arabian land level, a volcano level and a haunted hotel level and the Ruby chamber (mechanism) has a factory level, a frozen lab level, a toxic filled wasteland level and a pinball zone level. Each of the bosses and levels correspond with the chamber's theme without being too similar.
I just adore this game, Wario controls great, the levels are fun to play and explore and there's so much you can do in it. I don't know why I can't just openly say "THIS IS THE BEST GAME EVER" because there's literally not a single thing I don't like about it. It's that great.
2.
Metal Gear Solid 3: Made me realize my love for the stealth genre, especially when you actually try to hide and sneak instead of just playing it like a shooter and kill everybody, which you get punished heavily for later on the game anyway. It makes you appreciate the level design in the game. First of all, the story and gameplay compliment each other like bread and butter. This is probably one of the few games where I actually felt the story added something to the gameplay, and it's not exclusive to MGS3 alone and rather the whole series. The gameplay itself is smooth and Snake controls well, and the story adds to the tension that I feel is necessary for stealth. I've many things to say but don't want to make a short essay like for Wario Land 4.
3.
Rhythm Heaven Fever: Game is just full of charm, I love it. It's simple, but challenging and addictive. Really love this one, haha.