Most Popular
If you want to use the most popular Senna Top build, you should take Flash and Ghost as your Summoner Spells. For runes, you should pick Fleet Footwork as your keystone, followed by Presence of Mind, Legend: Alacrity and Cut Down in the primary tree, as well as Bone Plating and Revitalize in the secondary tree. For rune shards, you want Attack Speed in offense, Adaptive Force in flex and Health Scaling in defense, though it depends on the matchup. Finally, for your items, you should build Kraken Slayer as your first item, followed by Boots of Swiftness and Rapid Firecannon. For your last 3 items, you should build Infinity Edge, Blade of The Ruined King and finish with The Collector. For the skill order, you want to stick with Q - W - E. Right now, this build has 50.1% winrate over 349 matches played.
Optional
Your second option for Senna Top build would be to take Flash and Ghost as your Summoner Spells. The runes that this build takes advantage of are Fleet Footwork as your keystone, Presence of Mind, Legend: Alacrity and Cut Down in the primary tree, and Approach Velocity with Biscuit Delivery in the secondary tree. The shards used in this build are Attack Speed in offense, Adaptive Force in flex and Health Scaling in defense. The items for the build are Kraken Slayer, which you should build first, followed by Boots of Swiftness as your second item and Infinity Edge as your third item. The rest of the items in order are Rapid Firecannon, Lord Dominik's Regards and Opportunity. The skill order for this build is Q - W - E. In the current patch, this build has a 42.8% win rate over 138 matches.
Alternative
If you're looking for alternatives, you can use Flash and Ghost as your Summoner Spells, with runes being Fleet Footwork as your keystone, Presence of Mind, Legend: Alacrity and Cut Down in the primary tree, and Approach Velocity with Biscuit Delivery in the secondary tree, as well as Attack Speed, Adaptive Force, and Health Scaling as your rune shards. The items that this build incorporates are Yun Tal Wildarrows as your first item, then Boots of Swiftness as a second item and Infinity Edge as a third item. You want to finish the build with Rapid Firecannon, The Collector and Lord Dominik's Regards. As far as ability max order goes, you want to keep it Q - W - E. This build is currently sitting at a 51.9% win rate across 135 matches played.
Off-Meta
If you want to try an Off-Meta Senna Top build in the current patch, try Flash and Teleport as your Summoner Spells, Grasp of the Undying, Font of Life, Bone Plating and Revitalize as your primary rune tree, and Legend: Alacrity with Presence of Mind as your secondary tree. Use Attack Speed, Adaptive Force and Health Scaling as your shards. For items in this build, you want to complete Kraken Slayer as your first item, then finish Boots of Swiftness as your second item, and Rapid Firecannon as third item. The last 3 options are Infinity Edge, Blade of The Ruined King and Lord Dominik's Regards. The ability order for this build is Q - W - E. As of the current patch, this build has a 48.8% win rate over 129 matches played.
Situational Senna Runes and Items
Depending on the context of your game, your matchup, and enemy champions, you might want to consider building some situational items or take specific runes. Carefully examine your team, enemy team, game state , and your matchup to plan your Senna build accordingly! For example, when it comes to rune shards, you might want to take either Armor or Magic Resist, depending on which champion you are about to lane against.
Furthermore, if you want a more detailed breakdown, you should also check out Senna runes page.