Hiltl is the best with the widest choice but Tibits is good too though much less choice less choice. It is really worth trying Samses in Langstrasse. Decent food, nice buffet, vegan dishes etc. Not as posh or as huge as Hiltl but good value for money.
I know several restaurants in China that would beg to differ on the "Oldest in the world" claim.
That said (and not having tried Hiltl yet), Tibits is good and diverse, and there seem to be lots of Indian joints around which should be good for a veg option.
Maison Blunt is another option. It's near the Langstrasse, and it's Moroccan food, not solely Vegetarian but a lot of options and the food is really good.
I find the question to be very odd to be honest. Pretty much every restaurant and cuisine has standard menu options which have no meat in them and the idea of only being able to eat unsauced pasta or salad is not something I recognise in any place that I have lived for the past 10 years.
The only time I see occasional "slips" are when a vegetable soup is made with chicken stock or something like that - but the era of "yet another omelette" is long over.
Why make it political? Support restaurants that serve good food you like to eat - I'm fully behind the idea of vegetarians having access to good, interesting food wherever they choose to dine, but I don't see why it must be exclusionary.
same here, but i find the idea of a vegetarian or vegan restaurants good.
because you could extend that to why a bio market or bio farm? why this, why that? what i have heard when i spoke with veg restaurant owners (eg the only vegan hotel in germany so far in Konstanz) because people want to give a statement and that without the missionary-militant discussions. simply doing it by excluding animal produce from the menue. and for some guests it is important that the veg soup gets not done in a pan that cooked ochsenschwanzsuppe before.
from my view we eat way too much meat, dont know much (and even worse are not interested at all) about how the meat gets on your plate etc. it is time to think over and i love these restaurants.
Because OP wasn't looking for a treatise on the politics of veg, veganism or organic farming, but instead for recommendations for places where vegetarians could find reasonable options. It was specified that the restaurant didn't have to be all veg - and I believe in answering people's questions without a salting of politics. This isn't about eating meat - this is about finding veg food. There is a difference.
In regards to your larger 'why', it has to do with the economics of it - specialty restaurants cater to the minority, and are therefore limited in scope. If the objective is 'ensure vegetarians have access to reasonable dining options when they eat out', then the voting with the wallet you are suggesting works better when addressing the mainstream, rather than the niche.
Simply put, vegetarian restaurants don't need convincing, but 'regular' restaurants do. One method encourages all restaurants to be accessible to all diners - while the other punishes most restaurants for catering to the majority by serving meat.
See the difference there? Amongst other things, pushing the 'eat no meat' argument instead of the 'offer veg food' argument alienates many people and reinforces the perception of vegetarians as militant nannyish dietary killjoys.
[edit, additional] I agree too many people don't know/care about where meat comes from. They also don't know/care where their veg comes from, nor do they tend to look deeper into the big business 'certification' programs for organic, fairtrade or GMO free. Agri-industry is flawed in many ways, and knowing what's going on is important - for ALL sides.