I'm going to say the 2.5L MPFI since it's an evolution from the 2.5L Carb and 2.5L TBI. Honestly, I don't know why anyone would choose either of those. In which case the Pontiac 151 is a mean engine. You can build it up to some impressive numbers.
Although I have to say the 2.4L is an impressive package since you get a better transmission which makes for a relatively simple swap to a 4.0L as they use the same bell housing bolt pattern. The 6-speed is an awesome transmission that will hold up to something like 300HP and 300TQ.
Still for simplicity and just tooling around, there is something nice about the sound of a L or F head.