Depends on the sound you want, the guitar you have, and the music you play. Harder tension strings render brighter tones, and a bigger, fatter sound. Softer tensions strings are faster, allowing you more dexterity. It´s easier to play flamenco runs or baroque trills on the softer tension strings, but you sacrifice a certain amount of tonal quality. Savarez are good hard tension strings, but for my money, they´re a little expensive, and too much hard work. D'addario are O.K. I use La Bella regular 850 classical guitar strings, but I think the 950 Concert series are a little better. But try different brands and experiment. Getting there is half the fun.