My favorite 2D platformer is Duke Nukem 1 and 2. On the MS/DOS PC computers. Duke Nukem 1 and 2 lets you destroy almost anything like some walls, crates, and most of the enemies can be destroyed but not all of them.
This game is vary fair, while still giving you enough challenge, you have unlimited lives and chances to win this game, and you can literally fall from the top of the level and hit the ground without dying.
If you run into lava or spikes you only lose 1 unit of health off your health bar every second you stay in most hazards. there is no time limits either. The bosses in the 2nd Duke Nukem game are tougher then the first games boss fights were.
But once you kill the enemies they stay dead. (in both games) and will not re-spawn again. Until you start the level over again.
there is save points, (at the beginning of each level). However you cannot save on either game, during the levels. The first game will remind you to save your game. In a hallway. Before moving onto the next level. the 2nd game does not have a save hallway, but still lets you save at the start of each level. regardless
If you have a copy of Duke Nukem 3D the physical game CD. Then you have both of these 2D platformers
The Full Version of both Duke Nukem 1 and 2 can be found inside the "goodies" folder on all of the CD based Duke Nukem 3D copies.
My least favorite is just about all console platformers, for example I think Metroid would have been a fun game on the NES IF you could destroy the enemies, instead of having no weapons and needing to jump over the enemies, and Megaman 1 and 2 is just too tough of a 2d platformer game, in order to have fun with it
What makes a good 2D platformer in my opinion. Is allowing you to shoot duck, and blow stuff up, just like in Duke Nukem 1 and 2 and even in duke nukem 3D.
I Believe if you are ever limited in any of your combat, jumping, crouching, shooting, and heath and life capabilities. And on top that off the enemies constantly come back to life after you kill them and attack you
and you have a timer it makes me wounder why, I mean Duke Nukem 1 and 2 is just non-stop action and all systems go and you are not timed either