I asked this same question back in June.
http://www.winemakingtalk.com/forum/showthread.php?t=57055
It all really depends on a few variables:
1. What do YOU like?
2. How long do you want to age it?
3. What price range are you looking at?
For reds the answers to my question ranged from Eclipse Stag's Leap Merlot to Meza Luna Red to RJ Spagnols Winery Series Super Tuscan to a tweaked Fontana Malbec to WinExpert Sonoma Valley Pinot Noir to Showcase Merlot. Just FYI, there was a free kit giveaway a while back and it looked like 50% requested the Stag's Leap Merlot kit. Tells me something right there ...
Whites mentioned were Island Mist Peach Apricot Chardonnay and the Green Apple Riesling, WE white cranberry Pinot Gris, Winexpert California Mascato (if you like store bought sweet mascato this is a good kit), WE Luna Bianca, and Winexpert Eclipse Dry Creek Valley Chardonnay.
Hope this helps!