I have tried them all, and Levitra works best for me. All of them gave me headaches. 1/3 of Levitra gives me the lift with little or no headache afterwards.

I tried the full strength Cialis once, and as a "weekender" pill it worked, but that has its downsides too. Too much of a good thing.

Try various dosages (1/2, 1/4, 1/3 of a pill), and see what works best for you.

...I took the 20 mg pill (didn't know there was a 25mg) an hour before the session. I had the side effects. Stuffy nose, severe headache, and next day backache. They were bad enough but to make it worse twice I was left hanging (literally).

I switched to Viagra. It always works. I take 50 mg (cut the 100 mg pill in half). The side effects are much less severe and ibuprofen along with nasal spray take care of them.

My mistake. It is 20 mg tablets. I took half of one on a date Friday and it worked well enough. But then I found myself having 3 or 4 erections that night and the next day. Felt great but not so fun alone.  I had another date yesterday and took 100 mg Viagra and it worked perfectly. I guess I'm just going to stick with that since I don't know how long it will be for me between dates.  I have one this Friday so we'll see if it works good again. And I had zero aftereffects this time.

I've managed to buy generic forms of Viagra, Levitra, and Cialis in Europe.  I cut the 100mg V pills in half, which someone else upstream noted as well.  On the half does, I still get as hard as a rock (sometimes to the point of desensitization) and the side effects are manageable.  I might try 1/4 dose the next time and see what happens.  

I tried one of the 20mg Cialis pills.  The good news is that my erection seemed more natural.  I didn't get as hard as with Viagra, but all the sensations were  there and I performed normally.  But, the side effects were pretty severe.  I felt like I had the flu.  I haven't gone back to Cialis.  But, since I have a batch of them left, I may cut one in half and see what happens.

Generic Levitra was somewhere in the middle.  All things being equal, I will probably stick with the Levitra unless the 1/4 Viagra pills worked as I hope they do.

I've found that the tiebreakers are:  empty stomach, no alcohol, and stay hydrated.  

PS:  I haven't tried the gel versions yet, but I am told that they have less severe side effects than the pills.