The first 2/3 of Gardens of the Moon is meh, but the endgame is FUCK AWESOME.
I'm not even really sure how I'd rank all of the Malazan books. Each one has something cool about it.
Gardens of the Moon: suffers from throwing you into the middle of the action without explaining a lot of what is going on, but by the time you start to figure a few things out it becomes fuck awesome. Also, QUICK BEN.
Deadhouse Gates: Where the hell did Whiskeyjack go? Oh, never mind- COLTAINE. You also get to find out more about Fiddler and Kalam, which is cool.
Memories of Ice: Back to the Bridgeburners and Whiskeyjack. Probably the best book in the series but suffers a bit from the Mhybe storyline.
House of Chains: Karsa Orlong. 'nuff said one you get off your ass and read about him. Also introduces Tavore.
Midnight Tides: Serious departure from the other books- takes place on a different continent mostly in a flashback. But introduces perhaps the two funniest characters in a fantasy series- Tehol and Bugg.
Bonehunters: uneven but engaging. More Ganoes Paran and Fiddler. The battle at Y'Ghatan is probably one of the best in the series, which is saying something.
Reaper's Gale: even more uneven but still pretty awesome, because Tehol and Bugg are back, and Karsa Orlong has his showdown with the Emperor of a Thousand Deaths. "I will kill him- once."
Toll the Hounds: very well paced and written- back to the earlier high standard of the series in lots of ways, including a welcome return to Capustan, the setting of the first book. An absolutely cataclysmic fight at the end.