I can't recommend VVVVVV highly enough. The controls are tight and precise, the levels are well-designed, and the tone is wonderfully playful. There's plenty of challenge, though the hardest levels are optional. Frequent checkpoints mean you'll never have to repeat a long stretch of level a million times just to get back to the bit that keeps killing you -- you'll die many, many times going for some of the harder Shiny Trinkets, but you won't mind, because each time you'll be straight back into the action. VVVVVV is one of those games that leaves you with a warm fuzzy feeling when you beat it.