Thali is great. As someone primarily vegetarian I found it to be the best restaurant meal a person can expect to find in Doha. If you don't know thali is a usually lunch time meal platter consisting of a few smaller portions of Indian dishes for an unlimited thali where they food keeps coming, and sometimes thali is a larger platter without the unlimited situation.
Without further ado here are my top 5 thali joints in no particular order.
This is a Rajastani place with both limited and unlimited thali. During the weekday lunch time they have unlimited thali in either - chicken, mutton, or veg. 25 Qatari Rial
During the dinner time you can get a limited large veg only thali which can be shared between two people. 36 Qatari Rial
The phulka bread here is great and always made fresh. Rajastani people love ghee, give it a try on your phulka if you eat here.
Zaitoon is larger local chain with a few branches. I prefer the place on Salwa Road linked in the title. Here thali is only a lunch thing. Again this place offers chicken, mutton,or veg but you can choose a North or South Indian variety and it's limited. 18 Qatari Rial
The next offering is a pure vegetarian establishment which mostly does South Indian food, but I actually prefer their North Indian thali. They have specials which sometimes change but they almost perpetually have an unlimited North Indian thali for lunch. 18 - 20 Qatari Rial
For a South Indian craving I suggest the Rava Dosa here.
This North Indian place is probably the nicest atmosphere of the bunch. The food is pretty high quality but it's limited. 22 Qatari Rial
This final veg only suggestion is different from the rest. They serve other items sometimes, but they primarily do thali at all times and it's unlimited. Lunch thali here is priced at 20 Qatari Rial. Dinner and weekends at 25 Qatari Rial. The bread is really great, the salted peppers are super spicy, and the staff always try to overfeed you.