IMO 3rd Gen
Not sure about the 5th Gens and above, but my impression is big bloated and build for street cruising... and did I mention too much electronics! I bought my 3rd gen this Spring after much research--- simple is good, however, I required a tow vehicle which eliminated the 1st and 2nd gens. The 1st gen was also too simple (no ac-- we live in a HOT climate) among other things. 2nd gen has probably the weakest V6 the 3.0L had major problems... The 4th gen is great with a V8, however, I have several friends and workmates that have some and they report minor problems, poor mileage, some difficulty with maintenance, and cost. Overall their happy, but I have some doubt WRT longevity...
ON THE OTHERHAND, my 3rd gen is a balance between simplicity, functionality, economy (tons of affordable mod/parts options)(didn't cost me a ton of $$, just had to find one not beaten) and durability with strength (5000lbs. tow rating). Mechanically speaking, for a vehicle with a fair bit of electronics, its quite straight forward and easy to work on, overall. I've been a Chevy man in the past, but after working on my 4Runner (getting her tuned, maintained and upgraded) I'm blown away by the engineering, simplicity, elegance of design and durability of this truck! I find myself looking for an excuse to do something to it/ mod etc!!! I haven't been this excited about a vehicle since my 62 Chevy Nova rebuild! This truck will last forever, with the proper care!!