Here are my preferences based on the pens I have used.
Pens I have found that work flawlessly and have extra smooth nibs include the Waterman Medium carene and the Montblanc 146 Medium. Get the Medium nibs for a pleasant medium line and total smoothness.
If you go with a MB 149 and you want a firm and very smooth nib, get one made in the last ten years and get a Fine. It will write like a medium and be very smooth. People like to pick on MB but start talking to users and you will find that the pens work well and write smoothly.
I like Pelikans. The 800's I have have smooth nibs but not really glassy smooth. Some people want the nib to write like a soapy finger on wet glass, and others want it to write like a wet finger on smooth paper. Just depends.
The Pelikan 1000 nib is a lot more fun than the 800 nib. However, the 1000 nib flows like a demon, and if you get a Medium 1000 nib it will gush all over with even the slightest pressure. So if your hand in moderate to heavy touch-wise, this nib may well not be for you.
Duofolds are nice but nib smoothness varies greatly from pen to pen. I have eight of them and they have come with nibs that are wonderful all the way down to terrible nibs. On the other hand, I bought a duofold mosaic and the nib was really bad and I sent it to Richard Binder and it is now one of my favorite nibs. He made it very smooth and very tolerant of angle of attack, by which I mean I write fast and tend to turn the nib here and there when I write and so I needed a nib that was not fussy about the angle at which it was held.
The funny thing about having nibs smoothed is that the it does not seem to be a gigantic, costly effort and so one wonders why the **** fountain pen companies can't get it done better in the factor.
Bexley makes a smooth nib. I have had a few that skipped and had to go back but their service is good and fast. A smooth Bexley medium would be a really nice pen. I have an Americana and it is a dream writer.
The smoothest medium nib I have used is the steel nib made by Dani trio. I know this sounds crazy. It produces a full-bodied medium that is a little wet but it is very smooth on the paper and tolerant of which kind of paper you use, unlike say my Waterman Liaison nib that is smooth only on good paper. Maybe email Winedoc and ask what he has available with a steel nib? He may have a large alphaplus or some such pen kicking around for a darn good price with a steel nib. I am not affiliated with winedoc but am a happy customer.
A FIlcao Leader would be a good pen with a smooth nib. Great service from Filcao, and if you buy from Richard Binder he tunes the pen before mailing it.
Conklin nibs are usually glassy smooth. Some people have had issues with them, but I have one of their nozacs and it is glass smooth and works fine.
The waterman Phileas is a superb inexpensive pen with a smooth, full medium.
All recommendations based on my limited experiences.

Enjoy JC