Cubs have a long success with first baseman, but not many came via free agency.
Best: Carlos Pena
One might look at this and think, “Carlos Pena…really?” However, he really is one of the few notable first baseman that came in via free agency. Pena was only a Cub for one year in 2011, but he was one of the few things that season to watch. He was the “Adam Dunn-type” of hitter who would hit mammoth home runs, walk a lot, and strike out 160+ times. That is pretty much what he did for the Cubs.
Pena hit .225/.357/.462 with 28 home runs, an .819 OPS, 123 OPS+, 80 RBI, and 121 wRC+. When he got ahold of one, it went a LONG way. All things considering he was pretty fun to watch. Today he is an analyst on Marquee Sports Network.
Worst: John Mabry
We have to be somewhat fair here. Mabry was 35 and near the end of his career when he was signed by the Cubs in 2006 and had to step in after Derrek Lee broke his wrist. The veteran utility man had spent most of his MLB career with the Cardinals, and was hitting a very respectable .269/.325/.412 overall prior to his stint with the Cubs.
In his 107 games with the Cubs in 2006 he slashed just .205/.283/.324 with five home runs. He made just $1.075 million and left for Colorado the following season where he would play his final 28 MLB games. Not like this was a huge signing with big hopes, and he was brought into a bigger role at an older age.