So this will be my first trip to Cambodia arriving back end of November and I'm looking for a reliable taxi service from my hotel in Phnom Penh to hotel in Siem Reap any recommendations welcome.

Ratana Taxi was good for me

There are so many choices , who knows the situation whenever you arrive , but you won’t have to worry .
There will be some service . You might even want to share ..
Just be glad you’re able to taxi from there in Pp instead of from the new siem reap airport .
Maybe by then the situation won’t be all so brand new ..

Just ask your hotel, they all have contacts, never did it by taxi but i think cost will be 80 dollar or more. If you want to save some money, take EMS-post-minibus. Costs about 10 Dollar and good service, but they go from post-office to post office, what makes you need tuktuk from hotel to post office in PP and another-one from post-office SR to your hotel. All together maybe 15 dollar, depends where you stay. Best go PP-post office before to book a seat on the minivan. Nice Toyota minivans, with aircon and wifi, and running on time, whats strange in Cambodia !!

I would expect your hotel to add a premium on top of the actual fare, but as a tourist it may be easier.

You can try: 017 345 445 or 096 232 3334 for independent drivers if you're feeling adventurous, both drive Hilux's which cost a little more but are more spacious. I think I paid $70 for the whole car last time I went but it was a while ago and given LPG price increases it's probably more now. Don't have names sorry, both Bong to me. You may need a Khmer speaker, I have no idea what their English skills are like.

There is no such thing as a taxi "service" to anywhere in Cambodia. Someone may give you the number of an individual driver who has proved reliable. If you call that number, you may find that driver takes the booking but actually sends a different driver because he's busy. It will be good either way.
This may be a big ask for a new visitor but a good way to do it is to go to the place in PP where the taxis and minibuses mostly depart from for that destination. For SR that is the Japanese roundabout by the Tela gas station (just east of the Old Stadium on google maps). Look for a Highlander in good condition and make it clear you want a private taxi. I'd be looking to pay around $70 -( 80 max). Show the driver exactly where you want to go and there will be no extra charges. Alternatively you can buy a seat in a shared taxi ($20). Again, you can look for a good quality vehicle before you accept an offer. Shared taxis often are Toyota Siennas - nice vehicles. Reject all offers from beaten up old minibuses.I recommend noise cancelling headphones if you use a share taxi.
If you are a sole traveller, I strongly second the notion of using the Post Office bus. The tuktuk fares at either end (use Grab) will be minimal as the Post Offices are centrally located. Stops only at Kompong Thom PO.

For the return journey the place to go to get a taxi is on NR6 about 4km out of the centre, at the corner of the Old Bus Station Road (there is a Larryta office there).