What cities in Europe are good for solo travels?

Berlin, Amsterdam, Barcelona, Gdansk, Aarhus

What are your criteria? And are there any cities that are bad for solo travellers?

All of them.

Budapest, Sarajevo, London, and all those mentioned. Cities are an easy way to travel solo. Hostel prices and accommodations vary greatly so just read your reviews.

Under 30 single male:

Amsterdam, Prgue, Budapest Bucharest for one thing
Budva, Ulcinj for another thing.

Over 30 male:

London, Paris, Rome, Budapest, for one thing, Budva, Ulcinj for another thing.

London, Paris, Madrid, Barcelona, Athens

Almost any of them. The list that is already posted here is a good one.
Missing for me so far is one of my absolute favorites, Vienna. I have always enjoyed solo visits there. The only place I enjoy as much solo has always been Kowloon.... not so much in recent years though.
I would also add Munich. Solo in Munich is almost as good as solo in Berlin.
WRT Amsterdam, chose your late-night venues carefully.

WRT Paris, I have been there solo quite often and it is a clear favorite. There is so much to see and do. I would not leave it off of my list and especially if you have not been there often. You could spend weeks just in the Louvre. d'Orsay is a museum I have no problems visiting regularly. Do not overlook Paris and the surrounding areas.

Your question is an interesting one, because as others have pointed out, any city can be a good city for a solo traveler.

But I want to take your question as a challenge!

It seems like at the heart of your question is: What are experiences that are good for solo travelers, as opposed to being primarily skewed towards couples, families, and/or groups? And then which city seems to excel at those experiences? Ultimately you are the only one who can answer that for yourself. But I'll go ahead and go! Thus for me, I would lean towards museums, interesting bars, food markets, antique shops and flea markets. I would lean away from high-end dining, sightseeing cruises, "romantic" sites, and bachelor/bachelorette destinations.

As such, I might skip Paris, Rome, Venice, Prague, Florence, and Amsterdam, although ANY of those could also be great destinations for single travelers.

And instead, I'd focus on Madrid, London, Strasbourg, Cologne, Brussels, Turin, Milan, Stuttgart, and Berlin. It is an admittedly subjective (some might say arbitrary!) list, but that's what I came up with...

It's not the cities that determine good solo traveling; it's the traveler.

Portugal, Lisbon and Porto, have a good reputation for their hostels. A couple of them tend to be on the top 10 -15 lists for best hostels in Europe. Many in Portugal speak English, so language will not be an issue. Walkable cities and great public transportation. Not known for violent crime, but do have to watch for pickpockets. I've found the Portuguese warm and friendly.

I added a link to a YouTube channel where an individual just walks around and films. I think it's good in the sense that it gives you a feel for a place. You see what he sees. https://www.youtube.com/@ZenWalks/videos

It's not the cities that determine good solo traveling; it's the traveler.

Agreed. It's not the cities but the solo traveler. I've traveled to Dublin, London, York, Edinburgh, Amsterdam, Delft, Reykjavík and many smaller communities and I've never had bad solo experience.

My questions to the OP...
How well do you know yourself? Are you an extrovert or introvert? Do you thrive on being social? Are you comfortable socializing with strangers? Are you comfortable with solitude? It's a big part of solo travel and are you OK with the solitude without feeling lonely?

Would you consider a cruise?

I have sailed Norwegian going on my eight or ninth cruise. They offer studio cabins for one which include a lounge and activities, events for the solo passenger.

Last October, I sailed London (Southampton) to Lisbon and this October I am sailing the Mediterranean round trip Barcelona.

A cruise can be a very good deal.

Cruise is a good option. I like the idea of river cruises a bit more. A new city each day. But the time in the city is usually pretty limited.

Hey all! Just wanted to thank everyone in this thread for thought and enthusiasm that they've put into these recommendations and advise, it's all been very insightful and have taken it all on board.

In response to one of the above, in terms of my extraversion levels and comfortability with solitude - I'mperfectly comfortable with solitude, although to be honest, one of my main aims for a solo travel is to meet groups, make friends and socialise outside of my comfort zone, which I'm also perfectly comfortable doing. I'd like it to be as social an experience as possible, although I'm always happy and willing to get away by myself for a bit if I want to.

I am sure this is true of many locations, but one of the reasons I ended up living in Budapest was how easy meeting people was. Part of the reason was that I stayed close, but not in the tourist areas and did what locals do. Food, wine, laundry, coffee, breakfast. Then my first wife taught me to ask the name of everyone that served or helped me and to return to the good places and greet them by name. So, where ever you go, a good location in the city, a good attitude and maybe someone local to work you into a few local spots is helpful. Maybe More so than the city. I know have friends in a dozen cities that I stay in touch with and correspond with fairly often. For me the enjoyment is the people, not the church interior.

In Berlin the week markets on Saturdays and the flea markets von Sundays are very social events, also all kind of second hand shops, e. g. for music. Places like Books & Bagels - Shakespeare & Sons gives you somehow a guarantee to meet people and maybe exchange on an interesting topic in English language. Also the high number of events in Berlin are a perfect opportunity, e. g. in Kulturbrauerei but also street events like public raves, CSD or music festivals. Very special this summer in Germany is the European Football Championship with the fanzones and the public viewing places such as BRLO brewery - you will find tons of positive energy.

Maybe a place where the locals are more immediately open and not overrun with tourists. Roeun is good, or try Lyon.

Berlin would be great, but if you want to meet people stay in the fringes of thr tourist zones.

We don't know if the OP is travelling solo for a purpose or due to circumstance.
As I had previously noted, most anywhere is fine for solo travel. I have literally been around the world multiple times that way and rarely found myself in a place where I lacked companionship or experienced anything awkward or uncomfortable other then my first sauna in Scandinavia many years ago or the travel through East Germany to Berlin many years ago.

One thing I have always said is that I wish that I had discovered Ireland when I was in my early 20's. I stand by that. Cork, Galway, Dublin. all points in between. The weekend begins on Wednesday. Recovery is on part of Sunday and through Tuesday when a light warmup happens....and then there is Wednesday to start it all over again. As a solo traveler with an intent to have a good time, give me Ireland.