There are really only two options for visiting the park and Namibia in general. One option is to hire a car which is great because it gives you flexibility and allows you to go where you want when you want. This can be expensive unless you have a several people to split the cost.
The other option is to take a tour with an overland company. There are various companies that offer trip through Namibia. The one that I used, African Routes, was very reliable and a great value for my money. The guide and driver was perhaps one of the nicest guys I have ever met, and the itinerary was solid.
It took the 19 day Namibian Explorer that left from Capetown and ended in Victoria Falls. We spent three days in Etosha. I would have liked to stay more but it was the best option I had.
The cost is about $1000 US and includes many activities.
This park is one of the few ones in Africa where a self-drive without a guide can be really rewarding. This is due to the fact that animals congregate at waterholes, so if you have a map and know where the waterholes are, and if you have a bit of patience ... BINGO! Satisfaction guaranteed!
Just drive nice & slow, not only not to be a menace on wheels to the animals, but also to give yourself time to take in the wonderful scenery and spot the many interesting Etosha inhabitants ... whether furry, feathery or scaly :-)
Roads are in pretty good shape so no need for a 4WD, though a 4WD has the advantage of being higher, allowing for better views of the animals.
The best way to experience the Etosha National Park is by having your own car. Also normal sedan car is good enough as you are not allowed to drive off road anyway. The gravel roads are basically in good conditions but of course you have to drive rather slow first of all to be able to spot the animals and the second reason to avoid crashing into any.
Etosha Park can be reached either on a safari tour or by private car. There seem to be no public transportation; even the local workers are trying to get a lift to the gate or beyond, rather bizarre set up. Comparison between organized safari with backpacker firm to Sossusvlei for three days, double occupancy in a tent accommodation outside of Naukluft national park and three day safari with car rental and four star accommodation in Etosha park itself produces the result of 30USD difference a day; Etosha seems to be a great deal within Namibia itself pricewise and much better deal than Kruger Park in South Africa.
Any car can pass through Etosha. Rain season may make some areas more difficult to pass in a 2X drive, 4X4 is recommended.
Overland and open trucks are better because they sit higher and can see over a crowd of cars.
If you don´t have a car, the best way to visit the park is by an organized tour. I used Wild Dog and Crazy Kudu, and really enjoy the trip, very friendly and profesional staff.