The Lions had the 30th ranked offense, and the 32nd ranked defense. They could use an infusion of talent at nearly every position. However, the cupboard is not totally bare and there are a number of players that will be a part of the next rebuild. That said, the Lions have the #1 pick in the 2009 draft. What position to address? Which player to choose?
For a team to win you need many components. Good scheme, good coaching, usually you need a healthy cap situation (especially if you're building a team from the bottom up), and good players. You also need to field a team of 53 players total, with 11 starters on O and 11 starters on D.
The Lions have 56 players already under contract for 2009, plus they have the upcoming free agency and the upcoming draft. They need to fill some of the starting positions that currently do not have a starter in them due to that player's contract expiring. For example, they do not have a kicker. They do need to sign one, preferably Jason Hanson, who it sounds like will come back to the team. Another "hole" is at MLB. Paris Lenon is a free agent and I can not foresee him getting signed back up for the job. That does not mean that you take a kicker or a MLB first overall in the draft, you still need a scheme and coaches and to work within the salary cap, as well as build the team. It is irresponsible to sign a rookie MLB to a $60 million 5 year contract with $30 million guaranteed. That is more then any current NFL MLB makes. If you want to spend that much on a LB, get one in free agency, over pay him by 50% and you still won't spend as much cap as you would on one with the first pick in the draft.
So, you fill the MLB spot with either a FA or later draft pick. Period. And you do that after you have your coaching and scheme and plan in place.
Back to the Lions first draft pick, who to take. The salary cap dictates you will choose from QB, LT, DE, DT, RB, WR or if you really really stretch it to the max, CB. The last being such a stretch it probably won't and shouldn't happen. The Lions don't need a #1 WR, they need a #2. The Lions don't need a #1 RB, they need a #2. Many think that Avril and White can handle DE with a couple decent #2's coming in rotation. That leaves QB, LT, and DT.
One of those 3 positions will be the Lions first pick in the draft (barring a trade).
Does it matter which? Not according to the salary cap, no. Not according to a hole in the roster, since all three have players for that position, albeit not great ones. So, do you draft based on who you like? No. You base it on which of the players at those positions is the best and how that will translate into the NFL. The first being easier to figure out then the latter.
There are currently at least 3 names being bandied about each for QB and LT, and a couple for DT, though it's not expected that any DT will be deemed "best" when placed against the QBs and LTs coming out. Of course, there is still a coaching staff/scheme to get, and that could change matters, and there is still the combine, which will also alter things somewhat (that's where the mental aspect and the translation into the NFL guesses come into play).
You can't even go by history, as it shows many busts with the Juniors who come out at both QB and LT, many more busts then great players.
You can't even use the argument that you can get one later, since you can get a QB or a LT or a DT later. In fact, more QBs taken 3rd (in the 9th to 15th spots) have turned out better then QBs taken in the top 3, for example.
No, what needs and probably (hopefully) will happen is the candidates are rated, contract talks will begin, and of those who agree to a contract prior to the draft one will be selected, barring any trade offers. Whether it's a QB, LT or DT does not matter, any of the three will upgrade but one position on the team, a very important position, and will be in camp on time. That is what matters.
The rest of the draft AND free agency (along with coach selection and scheme implementation) is actually what will fix the most of the team, not just the first overall pick. That can only fix one spot, and only then if it's the right choice.