Does anyone know how to select a voltage for a transmission line for about 300 miles and 1000MW. I know it has to be either 345, 500 or 765KV, but which one is best?

Perhaps the answer to these questions can give you an idea of what you are asking for:
-Best depends on your system configuration.
Which one offers least losses on transmission lines and at the same time cost effective?
What level of voltage configuration exist in your country?
Can existing voltage configuration support the levels you are thinking about as per national standard?
What level of protection do you have for the system and the right of will ?
What level of funding is available for the project?

Thanks a lot Sir. I must say I admire your qualifications, I hope to gain some experience like you someday. I hope you do not mind connecting with me.

If you already have experience with any of these voltages in the country, or in adjacent areas, I would prefer that. It always causes additional complications to introduce a new (higher) voltage level.
If you still have a choice, you should select the level that will give you the best economy over the lifetime of the transmission line. That should include annualised capital cost, maintenance, transmission losses, cost of the required land, etc.

It will be the voltage of the closest substation that will allow the reliable interconnection of such amount of power. What are the characteristics of the existing substation, 300 miles away from your plant?

765 kV seems too high, unless you already have a 765 kV substation "waiting" to get one more line connected to it. A 765 kV line would be rated at more than 2,000 MW. Sounds like an "overkill" (very expensive) particularly if you do not have an existing substation at that voltage level. You would need a step-down transformer, added to the cost of the line.

Then, the additional question is the required reliability. Do you need to build a N-1 reliable system? In other words, can you afford loosing 1,000 MW of generation every time the 300-miles long line trips? See, if the line is being built in a region prone to lightning or other similar atmospheric phenomena, it might be prone to a few disconnections every year. Thus, you might have to build two lines (each rated 1,000 MW) to achieve N-1 reliability.

I'm quite curious: where is this generation unit located, that it requires a 300-miles long transmission line?

In Brazil, where we have this kind of problem (long distances for interconnection of remote hydro plants), we practically standardize it at 500 kV level. We have 765 kV for our largest plant (Itaipu) and some 345 kV (and even 440 kV) lines but the bulk of new transmission lines are being built at 500 kV level.

I think that the available voltage at the closest substation and redundance are important factors for the line, and perhaps the surge impedance loading of the line should be taken into account as well

i want to design a power transmision line between two cities...what must be the Frequency,voltages and currents required for it ? plzzz tell