Honestly, if you have someone who has worked with your property before and they were successful use them. If it's a different property then search around.
Used Ownwell last year and prior years I challenged myself. Their estimated tax savings was pretty accurate by the end of it so it seemed to be worth not having to do the headache myself. I may use them again this year but it depends on what I have going on. Get an extra $20 off by going here: https://www.ownwell.com/referral?owl=36DA7Z9B9
I've used Five Stone for the last few years. I think they are pretty popular in my area (183/Spicewood Springs) so they have a good amount of information to fight with. In talking with the tax consultant for our office, it sounds like last year's successful protests had a lot more to do with changing interest rates. I'm curious to see the appraisals this year.
This is the tax protest service I use. They still use the model of only charging if they save you anything. Used them the last few years and really like them.
If you have a good friend involved in real estate (i.e., a realtor), they have access to MLS and can gather some comps from your neighborhood for you. Use those to protest yourself in person. Take your friend out to dinner.