It really depends on what you want to do with it, is it for travel or home use? Many of us use a half cover for travel, they are lighter and don't take up much room. At home I throw an old sheet over the bike to keep the dust off of it, but some like a full cover for that. The cover I use when traveling is at the top left of this page:
Walmart has a full cover as well, I'm sure the quality isn't nearly as good as the Hondaline or some of the others available but at only $20 and for my purposes it's not bad. It comes with a storage bag and doesn't take up much room.
I work at an industrial site and use the Dowco Guardian E-Z full cover(GEZ-200, XXX large). This cover has a zipper at the rear which makes it really easy to get on/off. Also allows for quick access to bags/trunk. Can be put on hot pipes and completely covers the bike.
I also have the UltraGard touring half cover for the road.
I also have the Dowco XXX and can't say enough good about it. It is plenty big so you you can get it on and off easy, fits nice and looks great. The other thing is the company stands behind their product. I had a Dowco X and developed a small tear in the cover so I called the company up to see what they suggested to fix it and they said there is a lifetime guarantee so send it in, I did and they sent a new one out immediately. Great Service.
My wife made up a cover out of light polyester from the fabric store to throw over the bike while in the garage, it is very lite, easy on/off and stops the dust from getting on the bike. If anyone wants to know how she made it let me know.
I've got a Dowco Protector. It fits well and performs like it should. Got a great deal from Hal as I recall. It's been a couple years but it wound up costing about 50% of retail. I think he was having a sale.
) have had good luck with the Dowco Guardian,although the G-150 that i used on our GL1500 is a little snug on the 18. it was replaced once under warranty for Arizona sun rot,no questions asked. You have to ship it to them prepaid and they send you a new one in a week or so. The one we got about a year ago was thicker and more heavy-duty than the old one we shipped back. i use it every day at work because i dont have covered parking where i work. the Guardian line is pretty much waterproof. wouldn't leave home without it.IMHO.