This has been asked a million times, in a million threads on here, every time with arguments. Search a bit man!!! You've been around for 3 years, you can't use the noob excuse, you know better.
That said, your t-case is the only bit that hangs below your frame. Your rad tends to take the most hits. Your oil pan is critical. Thats why you'll get 10 different answers. I'd vote engine or rad.
Since you are too lazy to search for a question thats been asked a million times, I went ahead and did it for you. This took about a minute. Probably less time than it took to start this thread. If you put in 10 minutes, you'd pull at least 30-40 more threads on this question.
Nah... I'm knocking about on the ipad tonight and search on the app sucks. Plus I've been drinking so it was more of a beer-fueled question. But now that he links have been posted for me I'm set . Cheers.
If you can go for the full set. I would say gas tank is least important but the other four are definitely good to have.
But if you want to start with one or two go Rad and Engine then Trans. Start at the front and work back but you'll end up buying them all anyway (I did) so save yourself the shipping costs and do it in one lot.