The following are ensembles (except Highlander where Adrian Paul was clearly the star) without any real solid leads. Is Elena on Vampire Diaries considered its lead?
Here's a controversial one not many will agree on.
Leighton Meester on Gossip Girl
I genuinely liked the first three seasons and muddled through the rest. The conceit is over-privileged young people that try to act like adults. Most of the performances were bad (Chace Crawford was AWFUL) or mediocre (Blake Lively) or cheesy but awesome (Ed Westwick) but Leighton Meester really felt like a kid with a very old soul.
And then there's The Vampire Diaries. Before it got crazy and awesome in the middle of season two, Ian Somerhalder really carried the first and then half of the second season. It was great, whenever Stefan started acting like a tool, Damien showed up to clown him, smack him around and then maybe kill a totally innocent person. It'll never be the next Buffy or even Supernatural (It's closest to Angel with its many flashbacks and vampire mythology, but never reaches even that level), but I watch it to see what will happen next and because Somerhalder is clearly having a blast.
Not the lead, but Peter Wingfield as Methos really carried Highlander, even though Adrian Paul was a decent action star on it.