UPD: I have not touched T5 yet so I'm not mentioning it till I get to play it! That said, it's got some very favorable and well written reviews thus far, so go check 'em out!
Tropico the first's a classic. Having been a huge fan of Sim City and Tropico games (SC4/2000 and original Tropico) for years, I keep coming back to those games at least once in a couple of years just for nostalgia's sake.
Tropico 1 possesses that unique charm that nearly all the other installations, sadly, lack, alongside with some great tongue-in-cheek humor. I can say with certainty however that Tropico 4 more than made up for it and it's probably the only game out of the entire franchise worth purchasing (besides the very first one of course), seeing as T3 is just a watered down version of T4. Trust me, if you just skip on T3 you won't be missing ANYTHING. Kalypso really dropped the ball on that one.
So if you're looking to jump into the franchise, I recommend starting with either the 1st or the 4th installation, or better yet both. If you just want a fancy looking and relatively new city-building game in the spirit of SimCity, Anno and Cities XL, just skip this bundle entirely and buy T4.
Long story short: T1 started it all, T2 has pirates, T3 is an attempted but shallow reboot and T4 is an upgraded version of the previous game and what T3 should have been in the first place. Take your pick.