I've long been a fan of the AMS system used for Scottish Parliament elections - and still am when it's compared to Westminster's FPTP - but all the tactical nonsense this election has convinced me it's probably time to simplify things and go council-style STV for everything.
Big-party politicians like to pretend that single-vote systems are nice and simple, but really it just forces voters to play 3D tactical voting chess in their head for fear of wasting their vote - directly benefiting the established parties and obscuring people's actual preferences.
The one downside of STV is the risk that if folk want to vote for Party A (as opposed to certain individuals) they need to vote for all their candidates in exactly the right order, otherwise they risk spreading the vote and letting someone else in by accident. I think I'd be happy to allow party lists for that reason. People say that being able to express preferences for individuals is important to them, but if you object to one candidate on a list of three then maybe that party isn't for you! It would also get rid of those weird by-elections which just deliver the largest party another seat.
I notice that England is - infuriatingly - going the other way.