What is the best level to farm 1 ups?

The fifth stage of Nutty Noon gives you four one ups at the end, but only if you enter the tower through the secret door at the top of the first room, I think. It also loaded with orange stars, with four throughout the stage if you have already collected the energy spheres, and four more at the end hidden in metal blocks. Add the stars you get from the goal game, and you can easily get five extra lives each time you go through the stage. And you can get through the stage pretty quickly once you learn the mini-bosses' attack patterns and master using the abilities found in the stage.

Of course, it gets much harder in extra mode. You'll probably be losing a few lives just trying to make it to the end...

On normal mode do what kabii_sama said. On extra mode though, try getting a super ability. Most levels with a super ability allow you to earn GIGANTIC amounts of stars. Just pick out your favorite super ability, find an easy level that has that ability, then crush everything in your path. Stars=1ups. While it might get you less 1ups than the mini-boss tower, you'll probably lose half of the 1ups you're trying to get in extra mode.

I think you should go to level 1-4 the HAL room is there and you can find a lot of stars and 4 powers there is also a 1-up in there the HAL room is in a dark bush close to the end of the first room if dont find it you can check in the cheats section to know where it is

The best way to get 1UPs is just to play through the game. You get a ton of them if you survive through and explore every nook and cranny. Try not to risk some of them, though. Heck, while getting through *spoiler alert* Halcandra's Dangerous Dinner stages, I managed to rack up about 16 or so extra lives, giving me 22 by the time I got to Stage 4 of Dangerous Dinner, the boss fight. I suggest you try your hand at getting Gold or better in the Challenges, though - they'll give you a 1UP for a Gold, 3 for a Platinum.

Farming 1-ups? The game is just TOO generous with 1-ups, go around every level, you're BOUND to get about 25-40 1-ups.