I'll give advice from my experience. Obviously it's all an opinion and you might think otherwise, but here I go.
Liseberg
Absolutely lovely park, beautiful and a great atmosphere and location. Obviously Helix is fantastic, but I won't go in depth for that. Lisebergbanan and Balder are great backup rides. Don't fall for the Balder hype though, it's a very hit/miss coaster and it's best to go in with low expectations. I personally don't think it deserves the hype. Kanonen is alright, nothing special but not bad either. The rest is +2.
Mechanica and Uppswinget are both great flats and well worth your time. The rest is a bit mediocre though. Atmosfear was very disappointing when I rode it. Decent theming, a good build-up, great view, but it just lacks force. One of the weakest drop towers I've done. The other towers will be gone by next season. The water rides are both of very high quality and definitely worth doing.
The park is very easy to get to with public transport. From the Göteborg central station you can walk to Brunnsparken (nearby bus/tram stop) and take the tram to Liseberg, which takes like 20 mins. As for nearby parks, you can take a 3-hour train from Göteborg to Stockholm and visit Gröna Lund. It's a very unique and beautiful park with some great rides. I personally prefer the park itself over Liseberg but the latter has better rides. Gröna Lund has an amazing drop tower and S&S combo tower though.
Phantasialand
Probably the best park of the bunch, but I can't confirm yet until I've visited the others
. Black Mamba is definitely the standout coaster and a great and beautifully themed invert. Colorado and Winjas are both very good and two standouts in their coaster type. Night Hawk is not on the same quality but not a bad coaster. Obviously Taron will also be something if you visit next season!
Phantasialand only has two main flats, but they are both AMAZING. Talocan is pretty much the perfect top spin. There is not a single thing any other top spin I've done does better. Mystery Castle is not the most forceful drop tower, but a very unique take on the ride type and probably my fave along with the Gröna Lund towers. Again, Phantasialand has two fantastic water rides.
Going there by public transport is not as easy as the Swedish parks, but it's not hard either. There is a train from Köln that stops at Brühl. Then you can take the Phantasialand bus (€3 retour) to the park and back. As for nearby parks, There is Efteling and Toverland which are both great parks, but might be a little out of your range? Definitely worth a visit though, especially Efteling.
Europa Park
A massive and gorgeous park, nice atmosphere, and LOADS of rides. They have three standout coasters. Silver Star, when I rode it, was amazing with fantastic floater airtime on every hill. Easily the best ride of the park. Blue Fire is great, but Helix definitely overshadows it and I think it will move down in my rankings when I re-ride it this weekend. Wodan was good but nothing amazing. The other coasters range from decent to meh.
The water rides are nothing special and they don't even have any proper thrill flats. Dark ride-wise, it's nothing great either. It's definitely a park with more quantity than quality, but a brilliant park nonetheless. No comment on public transport, I'll have to see for myself this weekend. Tripsdrill is closeby which is low on quantity of major rides but is one of the most beautiful parks I've visited.