Spain and Paris Trip

a Eurail time.

Don't know what you mean by that. Eurail does not run any trains. I'd say fly to Spain, then buy train tickets from Spain to Paris. and fly home from Paris. There are currently 3 direct TGV's every day from Barcelona to Paris, taking 6 3/4 hours. You can probably buy tickets early next year.

Flying is likely your best option for getting to/from Europe. If there’s a direct flight, that’s the best, but if none is available, you’ll have to connect with an additional flight in North America, or in Europe. You might have a flight from the USA to Amsterdam, Frankfurt, or London for example, then on to Spain. Another possibility - you’ll want to check timing and prices - would be to get a round trip flight to/from Paris, and once you arrived at the Paris airport, connect onto a flight to Spain. Otherwise, fly in to Spain, fly home from Paris.

You can fly between Spain and France, or use the train. Unless you’re taking many train trips, individual tickets, rather than a Eurail pass, might be your best option.

regarding the best way for you to get to Europe and back to USA, it might depend on where you are. That's not clear from your post.

While most will fly and for them flying might be best, if you are near the right port and have enough extra time - perhaps retired like me - you might find that best for you is a ship - the Cunard ships for example have many crossings a year.

The devil is in the details. What part of the US would you be leaving from? Spain is a fairly good size country; what place (s) do you want to visit? One city or several? What do you define as best- cheapest, fastest, most relaxing, most luxurious, or something else?

AFAIK, the only way to travel between the US and Europe is by air or sea. However most cruise lines only do twice yearly repositioning cruises. The eastbound ones will be done before July, leaving only the regular runs by Cunard that go from New York to Southampton. You'd still need to get to Spain. And it would take at least a week. If time is a consideration, then flying is the only other option. Since we don't know your start or end points, no further advice is possible.

Your choice of travel to Paris is either train or plane. Hard to advise which without knowing details of your plans for Spain. The Man in seat 61 website may provide the information you need about train travel between the places in Spain and France. But you need to work out whether a Eurail pass (with added seat reservation fees where necessary) is better than just buying point to point tickets.

I didn't realise originally that this post was talking about potentially coming to Europe by sea.
But apart from cruise ships it is still possible, if a very niche market, to travel from New York, Norfolk, Savannah and Miami to Algeciras, Barcelona and Valencia by the French container ships of CMA CGM. Sailings are every 2 or 3 weeks all year.
I'm not going into much detail here as it won't be of interest to the OP but is an option.
These particular ships take just 6 pax although others take up to 12.

The question is the best way to travel to Europe and back to USA and within Europe

The 'best' way could be something like Netjet, Wheelsup , Vistajet or similar.

The 'best' way could be a sailing ship and trains/electric cars if carbon footprint is your greatest concern.