as you probably know, there is a whole genre of spaghetti western which Leone's stuff is only the most well-known in America... unfortunately I haven't seen much of any of them so I couldn't give you personal recommendations. However, there's a pretty decent 5-page article
HERE which talks about the genre as a whole and includes tons of titles you could check out.
also, they just showed Once Upon A Time in The West IN Monument Valley, which is where pretty much all of the classic westerns were shot (known for it's very unique mesa and plateaus)... i really wish i could've been there but oh well. you can read the aint-it-cool write-up
HERE.
Personally, I appreciate the sheer size of The Good, The Bad, and The Ugly, but think Once Upon A Time stands as the exact definition of the genre. Especially considering that Fistfull is basically a remake of Kurosawa's Yojimbo and Few Dollars More is basically a remake of Fistfull.
they are all fun though and, even though they are not "spaghetti," most of Eastwood's american Westerns are super badass awesome as well. Hang Em High, Outlaw Josey Wales, Pale Rider, and especially High Plains Drifter are all AWESOME. I haven't seen Two Mules for Sister Sara but hear that's also quality, if a little less badass.