What's the best M4 Kit available now?

If you can find one and don't mind handing over some big bucks, the DML M4A1 is available at times on Ebay and at model shows. It is very nice, but it is OOTB and therefore can be hard to find.

I'll give you a quick run down

Tamiya - M4, M4A3, M4A3 (105) all three of these kits are up to Tamiya's high standard, very easy to build & well engineered. All three do suffer from a common flaw though in that the bottom of the sponsons need to filled in. A template is available though to correct this and it is a fairly easy fix. It's available here: http://www.archertransfers.com/WEB%20pictures/ShermanFix.pdf

Dragon - their best IMHO, as Kaleu has already pointed out is the M4A1, but be prepared to pay through the nose. All of their Shermans are decent builds, some contain a large number of Italeri parts which makes them a little more challenging to build. Their M4A4's and Firefly VC can be found in many stores but are getting harder to find. Their E8's are also hard to find and these are the ones that contain Italeri parts.

Italeri - most of their Sherman products can be found with a little looking. Most of their product can be made with a bit more effort during the construction. Just be careful of two things 1) their tracks suck 2) a couple of their Shermans are mislabelled and describe the wrong Sherman variant.

Why not combine your two wants and do the M4A3E8? DML makes two variants of this kit, one is a late WW2 variant and the other is a Korean War variant. Both are essentially the same kit with different tracks. The late WW2 version contains some weld on armor plates and sand skirts.

Here are the older kit numbers, the re-releases have a different kit number in the 6000 range.
#9009 M4A3E8 "Easy Eight" Korean War "Rice's Red Devils" (new #6811)
#9010 M4A3 (76)W HVSS WW2 Sherman (FYI HVSS=E8 or Easy Eight)

Addtionally, DML also does a USMC Korean War variant armed with the 105mm howitzer and flame thrower called the M4A3 HVSS POA-CWS-H5 kit #6807. POA=Pacific Ocean Area; CWS=Chemical Warfare System (flame thrower); H5=105mm howitzer mounted in the Sherman turret.