Clover gets my vote. I have used many other brands, but Clover really stays sharp. Bluprint had them on sale.for $1.97...I bought six..gave three to my daughter. I should be set. BTW when I was at the cabin, had another brand that was dull. My husband took it to his workshop and used a fine sandpaper on it. Sharpened it up nicely....still using it there, but now have some Clovers to use instead.

I'm going to be controversial.

I think all seam rippers become dull over time. It's just the nature of the beast.
I have two methods of ripping seams:
My first method is I don't use seam rippers to rip seams, I use a razor or box knife. First I use the seam ripper to cut a couple of stitches, then I gently pull apart the seam. I keep some tension on the two fabrics by holding them apart and take the razor blade and touch it to the threads. This method works pretty fast.
My second method is if the seam I'm ripping isn't applique or cut on the bias, I use the Ricky Tims method. Grasp the two pieces of fabric and yank them apart. The seams will rip easily.

Someone had mentioned that they used a seam ripper and it tore a hole in her fabric. I'm going to hazard a guess that she was holding the seam ripper with the pointy end under the fabric. If you turn it over so the red ball is under the fabric and the pointy end is on top of the fabric, you won't get any ripping or tearing.

I went 40 yrs before discovering Clover. I sure wish I’d known of them way back. They are wonderful.