The standard is the
Calmark cover. I've tried several of the covers from the RV store made out of the inexpensive car cover type material. The best I've had is two seasons looking worn, completely shredded by the middle of the third. The Calmark I bought is in perfect shape approaching it's 8th season.
To be sure, it costs more, around 3 or 4x the cheap ones, so it needs to last 3 or 4x to break even. It appears to be doing that easily. One downside is that the fabric is heavier and real Sunbrella outdoor fabric, so it weighs significantly more than the cheapies, which makes it more work to put on and take off.