Indeed,Stewart is a fine actor. But there are many fine actors on TV. Many who were given material much better than Star Trek.
I believe that there were many instances of the excellence of Patrick Stewart in the material he brought to life as Picard. Such a product that very few actors to ever be televised could have performed as brilliantly. When judging an actor's worth relative to that of others in their medium, I believe the most important criteria is how important their involvement was to the quality & success of the product, & in TNG's case, his involvement meant absolutely everything.
Did the writing, premise, production values, & costars performances make for the best show on tv? Probably not, because in some cases it certainly wasn't. Does that mean that his performance should be valued less? I don't think so. In a way I think it should be valued more from the sheer adversity of that
Having said that, I'd never claim Stewart as tv's greatest actor ever, but I would grant him the praise of being one of the best to ever be televised, even with a character as uncommon as Picard, on a show as unorthodox as Star Trek TNG is to most other tv shows
Among that list, I'd also include actors like, Ian McShane as Al Swearengen in
Deadwood, Terry O'Quinn as John Locke & Michael Emerson as Ben Linus in
Lost, as well as perhaps Bryan Cranston as Walter White in
Breaking Bad
In an odd way, I feel the same about Patrick Stewart as I do Edward James Olmos. Was BattleStar Gallactica a show that fully depicted the quality of his acting talent? Not really, but is he still one of the greatest dramatic actors ever televised? Absolutely, & did we at least get to see some of that surface in his performance of Bill Adama? Yeah.... I kind of think we did