Some important questions for the OP that I think have some bearing on what is a good choice:
-How do you intend to vary it? Manually? Analogue signal? Digital signal?
-How good a sine wave do you need?
-Does the frequency need to be pretty stable/precise?
-Is startup/settling time a concern? (how much?)
-Any other constraints such as size, cost, time to make/design etc?
-If it's not a secret, what's this for?
From what little you've said so far I'd suggest either a function generator chip (IIRC these use deformed triangle waves), a reasonably high-speed microcontroller driving a DAC, or a beat-frequency oscillator, but I'm no expert on this. Others here would probably be able to give you more suitable suggestions if you can address these sorts of things though.