Return to the Temple of Elemental Evil can be fairly unforgiving: I was in a campaign where we completed a couple of sections (Nulb, the Moathouse) before hitting the first dungeon and only just escaping with our asses intact.
Indeed, two of the party were captured and sacrificed to the Elder Elemental God.
Recommendations for an adventure depend, I guess, on what level your party are at.
Starting levels:
Lost City of Barakus - has a large mix of urban, wilderness and dungeon adventure. The main dungeon is vast - the first level has a stack of encounter sites with kobolds, bandits, ghouls and a - ah, but that would be telling. As players head deeper into the complex, they discover that the city is a huge trap.
There's also
Wizard's Amulet (free download from Necromancer Games) -
Crucible of Freya -
Tomb of Abysthor if you want linked - and deadly - adventures against the Cult of Orcus.
I'd also recommend
The Whispering Cairn in Dungeon 124 - first part of the AoW Adventure Path (12 adventures taking brave PC's from novice adventurers to Epic-level heroes) which I consider to be the best first level adventure I've seen in a long while. It will finish with PC's at about 3rd Level, IIRC.
4th-Level or so:
Return to the Temple of Elemental Evil isn't bad, having played in it, but there is a lot of material to assimilate. I haven't got
Red Hand of Doom but it seems to be universally well-regarded.
The Grey Citadel is a good pick, there's urban adventure, investigation and a mammoth dungeon to explore. I'll just mention the supplementary adventure available
here for free which I wrote - it adds a little to the way of life (and death!) in The Grey Citadel and expands on the nefarious activities of one of the original work's villains.
6th-8th Level or so:
The Banewarrens wins here. All the material needed to run it is included in the book (you don't need Ptolus to run it - thankfully) and it has a number of difficult challenges. I'm already foreshadowing this one in the new Barakus campaign I'm running (and the PC's are only at L2).
Hope this helps.