NEW YORK (CNN/Money) - With the success of "The Sopranos" and "Sex in the City", HBO has long been the emperor of original programming on cable television.

But lately, the network just hasn't been able to churn out the type of ratings hits that put it on the map, giving competitors -- primarily Viacom (Research)'s Showtime -- a chance to make inroads with their own line of series.

Is it a serious challenge for the throne? Well, some think Showtime could put up a fight. But others declare in the land of Caesars, Showtime is still only a struggling gladiator. After all, the Viacom unit still significantly lags Time Warner (Research)'s HBO in terms of subscriber base and by extension ratings.

HBO boasts 28 million subscribers while Showtime has 13 million. And Showtime has had a tough time competing with HBO on the creative side in the past – relying more on programs aimed at niche audiences such as the gay community –think "The L Word" and "Queer as Folk" – and African-Americans – such as "Soul Food" – rather than shows aimed at gaining mass appeal.

And HBO isn't resting on its laurels by any means. The cable network is banking on its $100 million homage to the gladiators with the launch of "Rome" this weekend and its current programming, including "Six Feet Under," has garnered the network both critical acclaim and Emmy nominations.

But Showtime is looking to break out of its box this season with the launch of "Weeds" and controversial thriller "Sleeper Cell," due out later this year. And its comedy series "Barbershop", based on the popular MGM films, is also aimed at the mainstream.

"They're making strides in becoming more commercially viable," said Matt Roush, senior television critic at TV Guide. "If they can get one signature show, like HBO had with Sopranos, that'll put them on the map and get them the name recognition they need."

So far, Showtime has met with disappointment despite some high-profile series launches.

The comedy series "Fat Actress" won Showtime a solid debut with 924,000 viewers – a high level for a subscriber-based cable network of its size – but it wasn't able to sustain that momentum. "Huff" which garnered the network an Emmy nomination was renewed for a second season before its launch but pulled in abysmal ratings and "Weeds" has seen its ratings fall since its official launch, when the show attracted 488,000 viewers (538,000 during its sneak preview of the show).

Roush said there's some hope for "Sleeper Cell", which features a Muslim FBI agent infiltrating a terrorist cell in Los Angeles – a topical but controversial subject that could attract viewers.

"In the wake of 'Sex in the City' and with 'Six Feet Under' closing their doors, there's a sense that it's the end of an era at HBO," Roush said. "The perception is somewhat overstated but as HBO is held up on such a high pedestal, there's no where else for it to go but down."

And ratings have dropped across the board. According to Nielsen Media Research, the last season of "The Sopranos" fell to an average of 9.8 million viewers from 11 million in 2002. "Deadwood" averaged 2.4 million viewers, down from 4.5 million in its first season and the cult hit "Six Feet Under" pulled in an average of 2.5 million this season, down from 3.7 million last season.

But HBO spokesman Quentin Schaffer scoffed at the notion that HBO is off its highs. He said as a cable network, HBO isn't reliant on ratings like broadcast networks, which it counts among its competitors. Schaffer said the company gauges its success by a combination of ratings, critical response and the number of awards it receives.

"Coming up with The Sopranos may be a once in a lifetime thing," he said. "But we don't have to have another Sopranos because it's more important to us to have a wide range of niches."

Schaffer added that HBO continues to crank out the "water cooler shows" that generate the buzz and said the network beats Showtime in every metric from ratings to awards.

And analysts point out that HBO's sheer size and reputation may make it difficult for Showtime to compete on the same level.

"HBO took an early lead and Showtime has had to catch up," said James Goss, media and entertainment analyst at Barrington Research. "You can attempt to be the first choice but, with Showtime, they may want to attempt to be the most interesting second option."

He said HBO's current ratings decline won't dethrone the network but if the ratings continue to fall, down the road, it could affect its subscriber count -- a situation Showtime may want to exploit if the chance arises.

In the meantime,Showtime may stand to benefit from learning from HBO's example if it wants to be a viable option for viewers, said Brad Adgate, senior vice president of research at Horizon Media.

"Showtime is going to continue to try to compete with HBO and there's going to be some one-upmanship in trying to put out edgy innovative shows," he said. "If Showtime really wants to go after HBO more effectively, they can take a look at HBO's business model and look to broaden their entertainment to appeal to a wider audience."

A representative from Showtime wasn't available to comment.

HBO, like CNN/Money, is a unit of parent company Time Warner and a member of the writer's family works at Showtime.

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