Oppressor is very good due to the relevant PvP abilities in its tree. Most importantly, the ease of freezing someone by spamming Ray is giving more CC. In PvP CC is king, and also in PvP situations the burst damage on all trees is comparable.
On the forums, go down to the preview section. There is an entry in the forums there on how to copy characters to preview shard. Then you can test it on the test server which gives free respecs.
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Answered, and thanks for dropping by.
If you are strictly PvP, I'd say Oppressor.