Train, Rome

one of the largest train stations in europe, the roma termini is the main inlet and outlet, in and out of the city.
walking will suffice as well as the metro lines. if i recall, there are only 2-3 metro lines. but they are conveniently and strategically placed. during rush hours (8am-10am and 4pm-6pm) the metro is a mess and really inefficient. buses are abundant and convenient.

I dont know if there is a best way, but the way I did it was fly from Hahn, Germany with Ryan Air to Pisa for 30euros, where I took a 3 hour long train ride to Rome for another 30euros.
Use the Metro (the underground)

taxis are way too expensive

All paths lead to Termini (the main train station) including the new Leonardo Express train from the Airport.
There are two Metro Lines. Line A (red) runs East-West past the Spanish Steps and Vatican City while Line B (blue) runs North-South past the Colleseum and the Roman Forum. They intersect at the main train station, Termini.

The main airport is Leonardo da Vinci, also known as Fiumicino. The other airport is Ciampino, where most domestic, and some international, flights arrive. You can get a flight from just about anywhere in the world to Rome.

Buses run from Termini to cities throughout Italy. You can pick up a bus for towns in Lazio from various bus stops in the city: these buses run on the same tickets as city buses. There are regular train connections to all the major cities in Italy and Europe from Termini station. Trains are comfortable and fast.

The main road connecting Rome to the north and south of Italy is the Autostrada del Sole, which connects with the ring road circling the city.
Leonardo da Vinci (Fiumicino) airport is 26km (16mi) southwest of the city. One of the most convenient ways to get into town is by the Stazione Termini direct train, which usually runs hourly from the airport. You can also get a train from the airport to Trastevere, Ostiense and Tiburtina. A night bus runs to Stazione Tiburtina. If you're driving, an autostrada runs from the airport to the city via EUR - it's a 45-minute drive and will cost you a small fortune by taxi. Rome's other airport is Ciampino, about 20km southeast of the city. From there you can catch a COTRAL bus which connects with a subway to Stazione Termini, or you can drive down the Via Appia Nuova.

The city bus company is ATAC. Buses run from around 6am to midnight, with some services running throughout the night. The city's Metro service has two lines, both of which go through Termini. A bus ticket is also valid for the city's Metro and train services. You need to buy your ticket from a tobacconist, newsstand or vending machine before you get on the train or bus - there are hefty fines for travelling without a ticket, even if you are a dumb foreigner.

Most of the historic centre of Rome is closed to normal traffic, although you will be allowed to drive to your hotel. You'll need to get a parking permit from the traffic police if you wish to park anywhere in the centre, or you'll risk being towed. To rent a car you'll need to be at least 21 years old. If you organise your car in advance it will cost you less. There are several rental agencies for cars, motorbikes, mopeds and bicycles. If you'd rather leave the driving to someone else, you can pick up a cab from one of the city's many taxi ranks or phone one any time of day.

Isola Del Liri
By train:

Train Roma-Napoli via Cassino to Roccasecca.

Train Roccasecca-Avezzano to Isola del Liri.

By Autostrada (tollroad):

From Rome: A1(MilanoNapoli) exit Frosinone. S.S. 'dei Monti Lepini' towards Sora. Superstrada (Highway) Frosinone-Sora - exit Castelliri Isola Liri ovest.

From Naples: Al (NapoliMilano) exit Ceprano. S.S. 6 Casilina towards Napoli Crossroads to S.S. 82 'Valle del Liri' to Isola del Liri.

I got to Rome on a train from Paris. Paris Bercy, a train station nobody knew about, on the most ghetto SNCF train you've ever seen. Don't do it. Friends don't let friends ride couchettes =)

I think the best way to get to Rome is on an airplane. Business or First class if you need to cross an ocean.
Taxis are relatively cheap, but if you can figure out the bus system (just ask the front desk at your hotel) it's relatively efficient, and much cheaper too. I didn't find the subway terribly useful, until the last day, when I was trying to see everything instantly.

From any place in Europe by train you arrive at the Stazione Termini. Once you arrive you can check your bags at a baggage office and walk around outside the train station for a cheap hotel. Before I arrived in Rome I would hear that this area was very dangerous and for me , coming from New York it was just like the area around Penn Station in NYC. It wasnt that bad at all. Believe me...

The best way and probably the quickest is by train from the airport, it costs 16000 lires if you take the non-stop train to Roma termini in about 25 minutes, and 8000 lires if you take the stop train which arrives at Termini in aprox. 45 minutes. The only thing I forgot to do was to stamp the ticket before you go into the train, on the platform is a stamp. And when you arrive at central station, you can either take the bus or subway to the place you want to go, a day ticket costs 6000 lires.
By bus, cause you will see a lot more then by subway.