Spain/Portugal/Gibraltar - Recommendation for Connecting Airport In Europe to Barcelona

Next May I will be traveling with United from Portland, Oregon to Barcelona for a Mediterranean cruise. United doesn't fly
into Barcelona so I have decided to arrange the intra Europe flight with another carrier. I can fly into Heathrow (LHR),
Frankfurt (FRA) or Paris (CDG) for almost the same fare. I have also been investigating a miles + copay NC fare
and found a few in each direction at various airports. I have read some terrible reviews of long delays at Heathrow,
planes at Frankfurt landing out in remote airfields and people being bused in. Not too much yet about Paris. For those
of you who travel frequently and have experience with these airports I'd appreciate a little hand holding here in
regards to which airport you'd choose for ease of arrival/departure, upgrade potential, reasonable hotels nearby and
anything else you'd care to share with me.
I will not be able to book the round trip flight until early July and I want to be ready.

Thanks again to all who care to help me with this.

MUC and ZRH have the easiest connections in Europe.

Why not connect there?

FRA, CDG, and LHR are no bueno for easy connections.

United will book you on codeshare partners, so you can go through FRA, MUC, ZRH then onto LH or LX. You could also think about UA to MAD then Spanair. I also believe CO flies into BCN. Many many options!

Went via FRA in January. No problems and it works well with the UA schedule.

Went via MUC to BCN last year and it was very smooth. Best airport in Europe for connections IMO.

Out of those, I'd go with FRA.