I am going to assume that you're a free player, who is attempting to choose between the free end-game cruisers - the Assault Cruiser and the Star Cruiser.
The Assault Cruiser is a VERY sound choice - a bit of a jack-of-all-trades. It is definitely the end game cruiser for DPS.
However, the Star Cruiser is the superior support ship, with enhanced Science abilities. If you want to tank or leverage science abilities, the Star Cruiser is your ship of choice. Otherwise, I would recommend the Assault Cruiser.
Also note that you can purchase the Mirror Universe versions of these ships off the Exchange for relatively little compared to their cost in dilithium. Spending 200K EC is very cheap compared to 120K+ dil. But be sure you're getting what you think you're getting - the Mirror Star Cruiser looks like our Assault Cruiser (Sovereign class) and the Mirror Assault Cruiser looks like our Star Cruiser, so go by the name, not the looks. Their performance characteristics are very similar to their prime universe counterparts.
Considering the numerous options available at T5 in terms of C-Store and Fleet Ships, however, if you are thinking of actually purchasing a ship from the C-Store, there is a lot more to choose from, and a lot of caveats that go with the many choices. So as the previous poster said, we need to know more, because there is no "cruiser to end all cruisers."
Ya might want to claim the Breen cruiser from the winter wonderland before you start throwing zen into the c-store, the specs look pretty sweet, and it's free. I have the mirror star cruiser and tho it appears to be a good ship it suffers (like all fed cruisers) from a terrible turn rate that my max maneuvering captain and a pair of mk 11 RCS modules can't cure. It's no wonder it has an almost infinite supply (via BOFF station arrangement) of shield and hull heals.
Buy the ship off the exchange (for cheap testing) before burning a ship token or ZEN on this boat, you be glad you did.
For your last free ship, I agree with marc. Get a patrol escort or advanced escort, so you can still have a ship with lots of bang for your buck. However for when you aren't in the DPS race, I would also agree and say get the MUAC. It looks terrible (sorry, not a shovel fan), but it's identical to the standard AC in handling, BOff setup, and capability.
As c-store ships go though, get the regent. She's a beautiful ship, and has great combat potential. She loses out on tanking though... so if you want a c-store tank... I can't believe I am saying this... get and Excelsior R... That way you can also get the Fleet Excelsior later... Which currently is the best cruiser in the game for tanking and barely behind the Regent and Imperial for dealing damage.
The Odysseys look pretty, and those who have them swear by them, or hate them... but in all honesty, they are just vanity items. I own all 3, and live in one of them, she performs as well as I need her to, and her damage output is wonderful, but she's been out-classed by the Fleet Excelsior. Which in all honesty, as much as I hate to say it, is probably your best bet as Zen cruisers go.
But for freebie cruisers? Cough up 300k ECs and go get your MUAC.