It depends what game your playing really,
Gen 1 - The best way to level up in R/B/Y is to grind at the Elite Four, other than that you can always use the level up glitch but it may be hard to set up.
Gen 2 - Once again, grinding at the elite four in G/S/C is the best way other than using the duplication glitch on rare candies
Gen 3 - In R/S/E, you can use the bike and near the fiery path, there is a small ramp that the bike cant go up, simply hold up and you will be in a pseudo-loop while your Pokemon level up in the daycare. Or you know Elite Four lol.
Gen 4 - Elite four, that's about it
Gen 5 - Try to find and knock out audinos, they are the gen 5 version of chansey, also you can battle breeders in the stadium and they will give you rare candies and can be battled again and again.
Gen 6 - There is a loop in Lumiose City that can be used while your Pokemon is the daycare, just hold left!
Gen 7 - Try using Pokemon refresh and getting the Pokemon to full affection for more EXP, then use Roto Exp and battle the Eltie Four