While we've never been on a world cruise ourselves, we've sold a few of these and after living vicariously through our clients, it's definitely on our bucket list.
There are several to choose from and it really depends on where you want to go and what you want to see since the itineraries are quite different.
For example, last year we had a client who wanted to spend 6 months traveling around the world. Since that's not doable on any one ship, we booked them on a 3-month cruise around the world on one ship, then transferred them to a different cruise line and ship for another 3-month itinerary. The first one did a more northern route, while the second one did a more southern route. They only hit a port two times on a couple of occasions, but they were entirely different itineraries making for a wonderful cruise.
The best advice is to book early! These cruises tend to sell out and the earlier you book, the better the pricing and the better your choice in cabins. And obviously for something like this, you want a good cabin in a good location.
And speaking of cabins, that really is a personal choice. (What one person likes, another will hate.) It depends on what you want, what you're use to, and of course, how much money you want to spend. Can you take 3 months in an inside cabin? Do you want to pay a huge difference just to have a balcony?
Lots of things to consider. My suggestion would be to begin by listing all the places you've always wanted to see. Then make a list of ports that do not interest you. Then give these lists to your agent and let them do the research for you and present some options for your consideration.