Ooh .... I like this one.
I don't think that there's a single answer. Very very very few shows are both consistent and great.
In fact, if you restrict the sample size to shows that have run for five seasons or longer ... I think you'd run into some issues. Even some of the absolute best (Sopranos, The Wire) had a few seasons that weren't as stellar as the others.
That said, I would rather have a show that was great ... truly great ... for a little while than a show that just kept pushing out perfectly cromulent shows. But that also depends ...
For instance- Dexter Season 1 was groundbreaking (for the time). Season 4 was an all-time classic. Season 2 was really really good. And Season 3 was watchable. But the remaining seasons varied between "oh" (Season 5) to "please claw my eyes out" (Season 8), that the show itself became a laughing stock.
I also think we tend to de-value how hard it is to make a good and consistent product over a long period of time. The Shield, for example, was a quality show that ran for seven seasons. It's also one of the few shows that, arguably, had its best season as the last season. But more importantly, it didn't have any really bad seasons; even when it wasn't great, it was always good.