Of your list I have only done Rapperswil which I think is one of the most beautiful courses of the Ironman circuit. When you're up on the mountains it's just so beautiful.
The steps on the run make it more epic, lots of support there, it's actually fun. It's a beautiful race, well organized, you'll love it.
The downside is that Switzerland is expensive. But trains are always on time to the second so you don't need a car and can consider a wide range of towns and villages in and around Zurich. On the bright side Zurich airport is very well connected and the city is a few tram stops away.

UK races I'm not a fan. They are the toughest in my opinion, expect rain - but I'd take roasting heat over torrential rain anytime.

I can't speak of the other ones

I did Rapperswil 70.3 (aka “Rappi”) last year, I’m on a start line this year. It’s a local race for me, so I don’t look much at the elevation nor scenery. To be honest, it looks pretty normal for the surrounding area, for me.

The organization is tip-top. Support is awesome. The trains work so well, that I’ll just go 1h by train on the race morning. If looking for non-Zurich/Rapperswil accommodation, just check the train stations on maps.

I wouldn’t do the UK race. The others will give you a better experience.

I've done the UK Staffordshire race twice (third time on June 11th) and it is not in the slightest a 'destination' race. Stafford is a fairly humdrum town in the Midlands, the split transition is a pain, first 10 miles of roads narrow/rough and weather a lottery for the UK in June.

So pick from the others on your list....

My answer would have been different if we still had the quirky, brutal, majestic 70.3 at Wimbleball, Exmoor - but that race was binned when Stafford started (it really only worked for fields up to c1500).

Have you considered Challenge Family races? They're big in Europe. Some of them take place in truly majestic places. Challenge Geraardsbergen includes the iconic Muur de Geraardsbergen climb in Belgian Flanders. Challenge Wales has one of the most scenic seaside runs. Challenge Gdansk is easy to fly to and find lodging near as it's a seaside city-resort race; I've done it, it's a perfectly organized race with a rolling bike course passing through a tunnel under a river, and has the best finisher zone you've been to.

---

"FTP is a bit 2015, don't you think?" - Gustav Iden

https://www.k226.com/events/events.aspx

This is a really good site for listing all races.

There are lots of other brand races in Europe in June.

https://triathlon-stein.nl/

The stein 70.3 in south holland does not disappoint....fresh water swim, varied bike course with cobbled road villages like paris roubaix, hills.....super enthusiastic crowds, beautiful landscapes.....ex pro independant race organiser....super passionate.....enter for next to nothing .....unforgettable experience