L.B Cebik has a wealth of information on his web site:http://www.cebik.com
He says that if you only could put up one horizontal wire antenna, you should consider an 88' or 44' doublet. He also has articles on all of the antenna's mentioned in this forum.
I have found that seaparate antennas tuned to resonance for each band is the best, and (for DX) the lower the take off angle, the better. But that's not always practical.
I have used Extended Double Zepps tuned for a single band and up at least 1/2 wave to be the best horizontal antenna that is very easy to build - cheaply, and shows some gain over a dipole.
I have also confirmed that something like the Force 12 Sigma 40XK is an excellent DX antenna - but only if you stick to using it on a single band. Low take off angle - and because its a vertical tuned dipole, no radials and takes up little space. Force-12 has other great antennas like their Sigma 5. Cebik has an article called "Short hatted verticals" which explains why these work so well.
For 20 - 10, my MA5B minibeam, up 1/2 wave on 20M, far exceeded my expectations for DX work. Directivity helps a lot, and the MA5B is a real "compromise" beam, but a good one (although I really don't like traps).
If you mostly care about local (USA) work, then just about any doublet or long wire and a tuner will work "OK".
The Cebik web site is fantastic . .