All right, I wanna tell you all about a thing I found about 4 months ago and have been obsessed with ever since, and that's Copy Kitty.
What's Copy Kitty? Well, it's an amazing game created by Azure and Raibys that takes place in a robot-killing cyberspace simulation that tests the various abilities of the protagonist, Boki Lamira. These abilities include platforming (along with double-jumping and generally super smooth physics), kicking to destroy terrain, stealing weapons from fallen enemies, and even combining them! Weapons can be combined 2 or 3 at a time, resulting in almost 200 different world-destroying attacks! That's not even counting the several powerful single-only weapons and combinable flame/freeze effects that can be imbued on absolutely any weapon (individually or both at once).
The object of the game is to complete the various missions provided, which is done by exploring specifically-configured levels and defeating all of the marked Target enemies. There are other enemies that aren't Targets, but beating all the Targets is what finishes a level. The first missions are extremely simple, so it's easy to try out and get a feel for. But like with any game, the difficulty will increase the farther in you get. If you reach a point where that's no longer a suitable challenge for you, every level has 4 optional challenges you can complete that can sometimes be extremely difficult. These include: Best Time, Best Score, Most Enemies (kill everything, not just Targets), and Least Damage (usually 0 or very close, be warned). These challenges don't have to be completed all at the same time, and don't offer you anything more than accomplishment and a shiny gold smiley face on the level's icon, but they are extremely fulfilling. There's also a plethora of amazing bosses at the end of each world that are just as chaotic as the levels themselves. If you find them to be difficult, they're at least short while still being quite a thrill (which is also a nice thing about the tough challenges that require numerous tries).
If you get tired of Mission Mode for a bit, there's also Endless Mode, which has you fighting robots in waves that increase in difficulty until you die. It constantly generates terrain randomly, infinitely loops horizontally, and introduces a random bosslike upgrade of an enemy every 5 waves and changes the theme every 10, so the experience will always be a little different. A level editor is included, too! Almost anything you can find in a Mission Mode level, you can do for your own, and you can even create your own time/score/damage challenges! You can also download a ton of user-submitted custom levels (the files can be saved anywhere), save replays of your best runs, and there's a ton of extras, secrets, and surprises to be found. There is absolutely no shortage of content and replay value to be had here.
So if this sounds like it's up your alley, give it a try! It's a game that makes you enormously overpowered, but remains quite a challenge if that's what you go for, yet is fairly easy to pick up and play if you stick with the tamer stuff. It even comes with a tutorial to get you started. It also has a gorgeous soundtrack and visual style, although a very intense one, so you may or may not wish to turn the graphics down (which it lets you do right as you open it).
Here's a trailer, and you can find the game at the link below. The trailer's for Turbo Edition, which is $10 with more stuff, but the standard version is free and still very much a hugely complete experience on its own with a ton to do. It's not too big of a download, it's a quick and simple install, and the game itself doesn't require a lot of commitment as most missions are extremely short, so you don't waste a lot of time if this turns out to not be your thing. If you're still not sure, raocow has multiple playthroughs of the game, in which he has a jolly ol' universe-killing time.