A lens is the heart of a camera system. Yes, sensors play an important part of the overall resulting image as do your exposure settings but the lens is what determines what data the sensor records and the exposure settings act upon. It all starts with the lens.
Company's like Zeiss, Sigma and Tamron manufacture a line of high end lenses that rival or exceed those of similar purpose made by the camera manufacturers. Zeiss is a third party manufacturer that has done this for decades, and have a well respected reputation for quality and price. Zeiss lenses usually cost more than even the best lenses the camera manufacturers like Nikon or Canon sell. Sigma and Tamron are less than Zeiss and often less than a manufacturers equivalent but not always. Sigma and Tamron in the last several years are making lenses that exceed most manufacturers offerings. Today, there are several Chinese and Japanese "houses" building third party lenses for almost any brand. Quality is across the board, all over the place. Price is usually hard to believe. When I shot Nikon, I used several Sigma lenses that blew away anything Nikon made. They weren't cheap. The Zeiss lenses for Nikon (and Canon) are manual focus only though so as much as I would have loved to try the Zeiss product, my work really required auto focus capability so I never bought the Zeiss version of a lens. I have rented them though for personal use. They are very good. I have used other lenses like Voigtlander with excellent results but these are, for me, niche products.
When I decided to go all Fujifilm and switch from Nikon, it was because of the lenses Fujifilm makes. Fuji has been making lenses for a very long time and have built re-branded lenses for most camera company's at one time or another, under contract. Their cinema lenses are legendary. Fuji lenses are known for color rendering, and overall image quality including resolution but their build has always, always been top notch. Fujifilm as a camera company seems to be a lot like Leica to me. Leica builds fewer cameras but their lenses are legend and their camera's image quality (due in a large part to their lenses) is hard to argue with. Leica commands a certain price in the market and its an "organ selling" spot for most. Leica's business model also caters to a certain segment of photography. Its not a sports camera or useful for wildlife, but its perfect for people, events, journalism and things similar. I will be honest, I cannot afford Leica. Well, I could I suppose, if I wanted to but the times I have used one, I just wasn't as impressed with what I got out of the camera and lens enough to buy in. Fuji on the other hand did impress me. And Fuji's price point is way easier to absorb. I think Fujifilm's glass is about as good as it gets. Bar none. I will get some flack here but I stand buy it. No doubt, Fujifilm gets a lot of knocks because of it's decision on sensor overlays (X-Trans) and sensor size (no full frame, just kick ass APS-C and medium format offerings) but no one that has ever used a lens knocks the glass they sell. Indeed Fuji is the "poor" mans Leica. I guess. To me, its the better option since Fuji has a more filled out line of lenses to offer. But thats also a problem. Fuji's line of lenses as fine as it is doesn't have EVERYTHING. And Fujifilm being smallish in terms of a camera company, just has not garnered the support of Sigma and Tamron. They offer no mounts for Fuji and have no plans to. Which is a shame since a few products would be very welcomed. Zeiss does offer lenses for the Fuji X mount, three I think. They are great lenses too but only equal to the equivalent Fuji offering and the Fuji version is cheaper. There are a few Sigma lenses I WISH I could use on my Fuji's. There are also a few lenses from Nikon which I would like to see in a Fuji lineup, like a 200mm f2. I hear one may be coming from Fuji in 2018 or 2019. I could use it now. Yes, lens adapters abound but none for the Fuji mounts offer total control. No info is passed, its all manual control and in some cases, VERY manual to the point of guessing. Great for personal, slow work but not so good for commissionable work.
Fuji has an impressive offering of glass and right now between my X mount and G mount lenses, I have my work and personal needs covered well enough. Its not a perfect lineup but it works. I don't know if I could afford perfect anyway. In a way, I probably have too many lenses ( I have 12 X mount lenses, including a 8mm fisheye from Samyung and three G mount lenses) across my two systems. I'm seriously adding the wide G mount 23mm/f4 before January. It would be in my bag already but for the need for another X-T2 body. I do need to budget. And I will start to pair things down since my usage of my equipment is getting more refined which each month. I just hate to part with any glass though, because its all so good. and you never really know...well I suppose you do know. But....????