Most people will say just stick with a good long ester test such as test-e or a blend (short/long) like sust for your first cycle...... However, I advocate using deca along with the test on the first cycle for a couple reasons:
1) The deca will lubricate the joints really well which will reduce the chances of pain or injury caused by the unbelievable levels of intensity in your on cycle workouts.
2) You dont hear this much, but with deca, you have the ability to keep the gains better and sustain them. some will argue this is not true, but I believe the deca helps to solidify some of the gains made during the cycle. As with any cycle, keeping gains is largely contingent upon a well administered, intelligent PCT, but I believe the deca helps keep it more solid.
3) The test makes some users (especially first time) become overly energetic and maybe a little aggressive. Deca will help to even that out a little because it tends to make people a little lethargic when they use it by itself, the deca used with the testosterone helps to balance it out.