Take a look at the Venture 60 at www.btemodels.com. The "bte" in btemodels stands for Bruce Tharpe Engineering. Bruce was the engineer at Sig who penned the original design for the Sig Four Star 40 and 120. After leaving Sig Bruce started his own company and the Venture 60 was his first plane to go into production. It is very similar to the Four Star (which is a great choice) but it looks a lot more sleek and the quality of the wood is excellent since Bruce hand picks all the pieces himself. The whole story is on his web site.

I am in the process of completing my Venture 60 right now and have found it to be a very good build so far.

Of course, in answer to your question, this is just my opinion. You will get a countless number of answers since there are so many perfectly acceptible second planes around. You are better off sticking to some basic design characteristics when making your decision than trying to do what everyone tells you you should do. Here are some things to look for.

1. constant chord wing - this will result in a plane that is less likely to snap on you when you slow down too much.
2. semi-symetrical air foil - this will allow the plane to do more aerobatic maneuvers well than your trainer will allow but it won't be as radical as a fully aerobatic plane.
3. Low wing - you will not progress as far with your second plane if you go with another high wing plane. I realize a Cub has already been mentioned but technically this is more of an aerobatic trainer. No doubt they are beautiful planes - I just picked one up second-hand not too long ago and I love to just sit and look at it. Not that it would be a bad choice if that is what you really want - you will just be delaying the low-wing advancement till your third plane.
4. Tail dragger - you have to make the jump at some point. The way I look at it the earlier the better. Once you have done it you will find it isn't as hard as many make it out to be.

HMMMM.... I know I'm probably missing some items here but others will surely chime in and add to the list.

These basics apply to most of the planes already mentioned (except the Cub to some extent and the Kavalier which I believe has tricycle gear) so take them all into consideration and choose the one that looks good to you. Like I said, there are many good choices.