1994 SP Alex Rodriguez
The Alex Rodriguez rookie card in this set is one of his most popular and important rookie cards. From the time of it's release through the mid 2000s this card was highly though of, highly desired, and very valuable. I actually did not get this card at the time of it's release and instead picked one up years later. I don't remember the exact year that I ended up buying the set, but I know it was in the early 2000s and I paid a little over $100 for the set which included my copy of the Alex Rodriguez rookie.
Since his trade to the Yankees in 2004, ARod's career has taken a bit of hit for various reasons. Besides taking a dip on the field, the market for Alex Rodriguez cards has really tailed off. The 1994 SP set can now be found routinely for less than $50 and an ARod rookie card, which accounted for 90% of the $100 cost of my set, can now be found for under $30.
1994 SP Chan Ho Park
The set does have a few other nice cards, such as the rookie cards of Billy Wagner, Derrek Lee, and Chan Ho Park. The rookie cards in the set are generally in the first twenty cards of the SP set. The cards are apart of the Premiere Prospects subset which were foil cards and generally pretty condition sensitive, which helped drive some of the value of the Alex Rodriguez rookie card and some of the other decent rookies. High graded copies of the foil cards can run pretty high still.
1994 SP Die Cut Mark McGwire
The 1994 SP set also featured a die-cut version of each of the 200 cards in the set. The foil rookie cards are highly desired, but many collectors love the looks of these cards and they tend to be a pretty popular find. In many ways, I don't understand some of the grading on modern cards since the machinery used to print and cut the card is usually so precise that it is easy to find mint cards, but for whatever reason the die cut version of these cards can be a little bit tricky. I have found that the top inside corners can be tough.
While this is a great set and gets a great spot on my countdown, I would hold off on adding this set to your collection if you are looking to add the whole set, or just a few cards. Alex Rodriguez is signed through 2017 with the Yankees, and if trends hold true, the worse he plays the more this set will tumble in value. I would guess if I revisited this countdown again in another ten or fifteen years this set would probably be much lower. I would guess at some point the ARod rookie is a $10-15 card and the set is $25-$30.