EU elections use the D'Hondt system, which means that if, say, your goal is to keep UKIP from electing a MEP from Scotland, your vote is, in a sense, three times more effective if you vote for Green then if you vote for the SNP or Labour—watch the video or follow the links (
Wikipedia,
European Parliament) to understand why.
If you wish to vote tactically, you need to know how the parties stand in the polls. Below is data for Scotland from two polls on 13--15 May, spotted via
UK Polling Report. I'm voting my heart, with the Greens, but it's nice to know that it is also a tactically wise vote in opposition to UKIP.
| YouGov | ComRes |
| Labour | 31 | 16 |
| SNP | 30 | 40 |
| Green | 12 | 7 |
| Conservative | 10 | 21 |
| UKIP | 9 | 8 |
| Lib Dem | 7 | 5 |
| Other | 0 | 1 |