Tokyo Airporter is good. I used them once (when relocating here). Their fee of 18,000 yen is all-inclusive and they meet you at the customs exit. The driver was natively fluent in English. FYI, the toll to Shibuya is 2,550 yen and 2,350 yen to Shiba-koen. MK charges 21,500 to 22,500 all-in.

Also, NEX being much faster than a taxi is nonsense. Door-to-door taking a taxi is much faster even in traffic. You have to factor in getting down to the train, buying a ticket for a train that runs every 30 minutes and then getting from the station to your ultimate destination. A cab is faster even if you stay at Tokyo Station Hotel. You also get to see more, especially if you go via the Rainbow Bridge.

Before Xmas it took me one hour and ten minutes by car to get to NRT from Shibuya station area. And then the 3 was bumper to bumper all the way from Shibuya on-ramp up to Shiba-koen. Then NEX train takes 80 minutes to cover this journey plus I have add to 30 minutes to comfortably get from my house to the train.

But NEX is definitely cheaper for 4 people because the net cost is only 6000 yen if you do the NEX/Suica deal.