Depends on how soft you're talking. I personally want the softest tip possible so I use the Kamui Black Super Soft. I've tried the Tiger soft and the Moori soft as a few other brands and non of them are as soft as the Kamui Black. Of course it's still not soft enough for me. I wish I could get a marshmallow to stick to my cue so I could play with that.
Oh, and another great thing about the Kamui is that it doesn't mushroom. I've had 3 now and I've never had to burnish them after they get installed. They hold their shape just awesome too. Oh, and they hold chalk great. Yep, they are the best tip on the market IMO.
Yes, they will mushroom slightly when brand new, but what I like to do is play a couple racks of "slam ball" (practicing your power stroke on every shot) to intentionally compress them, and then I shape them back to normal (remove the mushroom/bulging part) with the fine grit side of an emery board (grit-coated cardboard slab used to shape ladies' fingernails):
After that initial "preparation" with a couple racks of slam ball and emery board correction, the tip doesn't mushroom anymore.