T-Joint

I have to connect two cylinders (pipes). It looks good with boolean (Union). But when I do smoothing, it becomes terrible. What is the best way to connect in such cases?
1 2 3

there is an addon for pipes
in mesh extra and it has all these connections included

happy bl

Yeah, booleans do that. Try enabling auto smooth, or if that looks too sharp for you, add a bevel modifier.