I haven't played many. I tend not to be too fond of WRPGs... But NWN was playable and even fun with 100% turn-based, rules enforcement on. It was completely unplayable with the default RTwP. I don't have enough data for "best" though, sorry.

Lords of Xulima?

From what I've played, Fallout 1&2.

Jagged Alliance 2 also has strong RPG elements, so definitely worth a mention as well.

The original Fallout games are good because of their setting and quests, but the turn-based combat in them is pretty weak imo. You don't really have many options in it, it's a very basic system.
Probably can't comment, because I haven't played enough games, but personally I really liked the turn-based combat in Age of decadence and Expeditions: Conquistador.

Hang on ... there was a turn-based combat option in NWN? The first one? I tried it a few years back and quite strongly disliked the real-time combat, but didn't realize it could be changed.

Dark sun: Shattered lands is also decent.
Haven't played it myself, but Temple of elemental evil is widely considered to be a great implementation of D&D rules.
I also really like Albion, more so for its story and general atmosphere, but it also has ok turn-based combat with a lot of spells.
tbh I really think the turn-based combat in the Fallout games is pretty weak compared to those other games, because you have so few options (basically just shooting, or maybe a targeted attack). You need a wider range of attacks or spells for an engaging system.

Are we talking best game overall among those that happen to have turn based combat, or the game with the best turn based combat? It's an ok game, but the grind gets ridiculous later on.

I had in mind best WRPGs that happens to have turn-based combat, but I'd be interested to hear about the other way as well :-) I have played ToEE quite a bit. The combat system is probably the best turn-based implementation of a D&D system that I've seen. But, unfortunately the campaign they built on the engine is really poor. I have never been able to finish it.

Yeah, the combat in the original Fallouts is fairly simplistic. Either you have the firepower/armour, or you don't. I guess those games were good because of other factors, like the freedom, openness, meaningful choices, etc.

Yeah, the question really needs to be more clear...best game overall, or best turn-based combat?
Another recent game is Underrail, which is widely considered mediocre in the story department, but absolutely stellar in its systems and turn-based combat. So it depends what one wants from an rpg.

Overall, I'd say Fallout is a strong contender, but I actually don't like the combat there all that much. Shadowrun Dragonfall has good balance of being a really great game overall, and having fun combat. Combat is pretty much exactly the same in all the Shadowrun games, but Dragonfall is the only one with really great characters and story. I loved the combat in Divinity Original Sin, it might have my favorite turn based combat in an RPG in fact, but the story and characters were bland and utterly forgettable. Though I did like the world and graphics a lot. Strangely enough I did not enjoy Original Sin 2 at all, and quit very fast. Blackguards has good combat, or rather it would have if it were not so dependant on RNG.

Would HoMM 4 count? I mean, I know it's usually classified as strategy, but 4 is much heavier on the "Heroes" aspect, with them being actual units in combat, and having multiple heroes in one army. In fact, if memory serves, The Gathering Storm expansion plays out a lot like an RPG, with final campaing being pretty much a party of charachters from previous campaing teaming up and working pretty much exactly like any adventuring party would.

Yerch, the combat in Underrail is not even mediocre!

Wizardry 8

Another recommendation: Legends of Amberland: The forgotten crown. Fantastic turn based combat, I really love it.

I haven't played Underrail myself. But I remember you complained bitterly about Expeditions: Conquistador ("too difficult, uninstalled") which imo had great turn-based combat, so sorry, I can't trust your judgement :-)